Update DIRECTORY.md

Co-authored-by: Amos Paribocci <aparibocci@gmail.com>

Author: domix80

DIRECTORY.md

@@ -153,8 +153,6 @@
     * [Sol2](https://github.com/TheAlgorithms/Ruby/blob/master/project_euler/problem_004/sol2.rb)
   * Problem 005
     * [Sol1](https://github.com/TheAlgorithms/Ruby/blob/master/project_euler/problem_005/sol1.rb)
-  * Problem 006
-    * [Sol1](https://github.com/TheAlgorithms/Ruby/blob/master/project_euler/problem_006/sol1.rb)    
   * Problem 020
     * [Sol1](https://github.com/TheAlgorithms/Ruby/blob/master/project_euler/problem_020/sol1.rb)
   * Problem 021