Update app.py

Amended the docstring in count_letter

app.py

@@ -13,8 +13,8 @@ def letter_count(letter:str, word:str)-> int: #it's import to specify the return
     #Keep this format for the description / args / args description but feel free to modify the tool
     """A tool that counts the number of times a letter appears in a word
     Args:
-        arg1: the letter to count
-        arg2: the word that contains the letter
+        letter: the letter to count
+        word: the word that contains the letter
     """
     return word.count(letter)