Update 79.word-search.md

fix : add return False

Author: rejoicesyc

problems/79.word-search.md

@@ -217,6 +217,7 @@ class Solution:
                 if board[r][c] == word[0]:
                     if dfs(board, r, c, word, 0):
                         return True
+        return False
 ```
 
 _Javascript Code_ from [**@lucifer**](https://github.com/azl397985856)