Fixed git-push manpage

In git-push it is the remote repository and not the
local repository which is fast forwarded. The description
of the -f option in the git-push manpage gets it the other
way round.

Signed-off-by: Jyotirmoy Bhattacharya <jyotirmoy@jyotirmoy.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-push.txt

@@ -79,7 +79,7 @@ the remote repository.
 
 -f, \--force::
 	Usually, the command refuses to update a remote ref that is
-	not a descendant of the local ref used to overwrite it.
+	not an ancestor of the local ref used to overwrite it.
 	This flag disables the check.  This can cause the
 	remote repository to lose commits; use it with care.