updating DIRECTORY.md

Author: anurags10

DIRECTORY.md

@@ -22,6 +22,7 @@
   * [Rat In Maze](backtracking/rat_in_maze.py)
   * [Sudoku](backtracking/sudoku.py)
   * [Sum Of Subsets](backtracking/sum_of_subsets.py)
+  * [Word Break](backtracking/word_break.py)
   * [Word Ladder](backtracking/word_ladder.py)
   * [Word Search](backtracking/word_search.py)