Documentation: git-rebase -C<n>

Replace -CNUM in Synopsis section with -C<n> to make it consistent with
the description text.

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

Documentation/git-rebase.txt

@@ -7,7 +7,7 @@ git-rebase - Forward-port local commits to the updated upstream head
 
 SYNOPSIS
 --------
-'git-rebase' [-v] [--merge] [-CNUM] [--onto <newbase>] <upstream> [<branch>]
+'git-rebase' [-v] [--merge] [-C<n>] [--onto <newbase>] <upstream> [<branch>]
 
 'git-rebase' --continue | --skip | --abort