updating DIRECTORY.md

Author: github-actions

DIRECTORY.md

@@ -335,6 +335,7 @@
  * [ReverseWords](https://github.com/TheAlgorithms/Javascript/blob/master/String/ReverseWords.js)
  * test
  * [CheckAnagram](https://github.com/TheAlgorithms/Javascript/blob/master/String/test/CheckAnagram.test.js)
+ * [CheckFlatCase](https://github.com/TheAlgorithms/Javascript/blob/master/String/test/CheckFlatCase.test.js)
  * [CheckPalindrome](https://github.com/TheAlgorithms/Javascript/blob/master/String/test/CheckPalindrome.test.js)
  * [CheckPangram](https://github.com/TheAlgorithms/Javascript/blob/master/String/test/CheckPangram.test.js)
  * [CreatePermutations](https://github.com/TheAlgorithms/Javascript/blob/master/String/test/CreatePermutations.test.js)