updating DIRECTORY.md

Author: pre-commit-ci[bot]

DIRECTORY.md

@@ -1260,6 +1260,7 @@
   * [Can String Be Rearranged As Palindrome](strings/can_string_be_rearranged_as_palindrome.py)
   * [Capitalize](strings/capitalize.py)
   * [Check Anagrams](strings/check_anagrams.py)
+  * [Count Vowels](strings/count_vowels.py)
   * [Credit Card Validator](strings/credit_card_validator.py)
   * [Damerau Levenshtein Distance](strings/damerau_levenshtein_distance.py)
   * [Detecting English Programmatically](strings/detecting_english_programmatically.py)