Documentation/git-whatchanged.txt: show -<n> instead of --max-count.

... to match the change we did earlier to git-log documentation.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-whatchanged.txt

@@ -27,7 +27,7 @@ OPTIONS
 	output format that is useful only to tell the changed
 	paths and their nature of changes.
 
---max-count=<n>::
+-<n>::
 	Limit output to <n> commits.
 
 <since>..<until>::