Doc: Fix spelling errors of 'initial' in enum.py (GH-16314)

Lib/enum.py

@@ -672,7 +672,7 @@ class Flag(Enum):
         Generate the next value when not given.
 
         name: the name of the member
-        start: the initital start value or None
+        start: the initial start value or None
         count: the number of existing members
         last_value: the last value assigned or None
         """