git log documentation: teach -<n> form.

We say "this shows only the most often used ones"; so instead of
teaching --max-number=<n> form, list -<n> form which is much
easier to type.

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

Documentation/git-log.txt

@@ -27,7 +27,7 @@ OPTIONS
 
 include::pretty-formats.txt[]
 
---max-count=<n>::
+-<n>::
 	Limits the number of commits to show.
 
 <since>..<until>::