Update first_character_that_repeats.py

kyu_6/first_character_that_repeats/first_character_that_repeats.py:8: error: Syntax error in type annotation  [syntax]
kyu_6/first_character_that_repeats/first_character_that_repeats.py:8: note: Suggestion: Use Tuple[T1, ..., Tn] instead of (T1, ..., Tn)

kyu_6/first_character_that_repeats/first_character_that_repeats.py

@@ -5,7 +5,7 @@
 """
 
 
-def first_dup(word: str) -> (str, None):
+def first_dup(word: str) -> str| None:
     """
     Find the first character that repeats
     in a String and return that character.