docs(repository.diff): fix docstring keyword 'flag' -> 'flags'

The BaseRepository.diff signature takes 'flags: DiffOption = ...', not
'flag', but the docstring's Keyword arguments block listed 'flag'.

Closes #1138

Author: MukundaKatta

pygit2/repository.py

@@ -614,7 +614,7 @@ def diff(
             If 'b' is None, by default the working directory is compared to 'a'.
             If 'cached' is set to True, the index/staging area is used for comparing.
 
-        flag
+        flags
             A combination of enums.DiffOption constants.
 
         context_lines