updating DIRECTORY.md

Author: github-actions

DIRECTORY.md

@@ -204,6 +204,7 @@
   * [Problem01](https://github.com/TheAlgorithms/Java/blob/master/ProjectEuler/Problem01.java)
   * [Problem02](https://github.com/TheAlgorithms/Java/blob/master/ProjectEuler/Problem02.java)
   * [Problem04](https://github.com/TheAlgorithms/Java/blob/master/ProjectEuler/Problem04.java)
+  * [Problem06](https://github.com/TheAlgorithms/Java/blob/master/ProjectEuler/Problem06.java)
 
 ## Searches
   * [BinarySearch](https://github.com/TheAlgorithms/Java/blob/master/Searches/BinarySearch.java)