Document -<n> for git-format-patch

The -<n> option was not mentioned in git-format-patch's manpage till
now. Fix this.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-format-patch.txt

@@ -50,6 +50,9 @@ OPTIONS
 -------
 include::diff-options.txt[]
 
+-<n>::
+	Limits the number of patches to prepare.
+
 -o|--output-directory <dir>::
 	Use <dir> to store the resulting files, instead of the
 	current working directory.