Exception in thread "main" error : conversion = '%'

 java.util.UnknownFormatConversionException: Conversion = '%'                
        at java.util.Formatter.checkText(Formatter.java:2579)                                                               
        at java.util.Formatter.parse(Formatter.java:2565)                                                                   
        at java.util.Formatter.format(Formatter.java:2501)                                                                  
        at java.util.Formatter.format(Formatter.java:2455)                                                                  
        at java.lang.String.format(String.java:2940)                                                                        
        at com.teamtreehouse.Treet.toString(Treet.java:18)                                                                  
        at java.util.Formatter$FormatSpecifier.printString(Formatter.java:2886)                                             
        at java.util.Formatter$FormatSpecifier.print(Formatter.java:2763)                                                   
        at java.util.Formatter.format(Formatter.java:2520)                                                                  
        at java.io.PrintStream.format(PrintStream.java:970)                                                                 
        at java.io.PrintStream.printf(PrintStream.java:871)                                                                 
        at Example.main(Example.java:17)  

Example.java

import java.util.Arrays;
import java.util.Date;
import com.teamtreehouse.Treet;


public class Example{

  public static void main(String[] args){
    Treet treet = new Treet(
      "eugeniovillarreal",
      "Twitter confirms it’s testing a feature that allows users to more easily " + 
      "create “tweetstorms” – those series of connected tweets that have grown to be a popular" +
      "workaround for Twitter’s character count limitations.",
      new Date(1447979631L)
    );

    System.out.printf("This is a new Treet: %s %n",treet);

    Treet secondTreet = new Treet(
      "bobbyblue",
      "ATwitter confirms it’s testing a feature that allows users to more easily " + 
      "create “tweetstorms” – those series of connected tweets that have grown to be a popular" +
      "workaround for Twitter’s character count limitations.",
      new Date(1511550530L)
    );

    System.out.printf("This is a new Treet: %s %n",secondTreet);
    System.out.println("The words are:");
    for (String word : treet.getWords()) {
      System.out.println(word);
    }

    Treet[] treets = {treet, secondTreet};

    Arrays.sort(treets);
    for (Treet exampleTreet : treets){
      System.out.println(exampleTreet);
    }

  }
}

Treet.java

package com.teamtreehouse;
import java.util.Date;

public class Treet implements Comparable{
  private String author;
  private String description;
  private Date creationDate;  


  public Treet(String author, String description, Date creationDate){
    this.author = author;
    this.description = description;
    this.creationDate = creationDate;
  }

  @Override
  public String toString(){
    return String.format( "Treet: \"%s\" by %s on s%",
                         description, author, creationDate);
  }

  @Override
  public int compareTo(Object obj){
    Treet other = (Treet) obj;
    if(equals(other)){
      return 0;
    }
    int dateCmp = creationDate.compareTo(other.creationDate);
    if (dateCmp == 0){
      return description.compareTo(other.description);
    }
    return dateCmp;
  }

  public String getAuthor() {
    return author;
  }

  public String getDescription(){
    return description;
  }

  public Date getCreationDate(){
    return creationDate;
  }

  public String[] getWords(){
    return description.split("[^\\w@#']+");
  }
}