signoff-option: linkify the reference to gitfaq

The GitFAQ is a proper manual page in the section 7, so refer to it
using the usual linkgit:stuff[7] syntax.

Helped-by: Kristoffer Haugsbakk
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/signoff-option.adoc

diff --git a/Documentation/signoff-option.adoc b/Documentation/signoff-option.adoc
index 9a80d60..6fc2769 100644
--- a/Documentation/signoff-option.adoc
+++ b/Documentation/signoff-option.adoc
@@ -19,4 +19,4 @@
 +
 Git does not (and will not) have a configuration variable to enable
 the `--signoff` command line option by default; see the
-`commit.signoff` entry in the gitfaq for more details.
+`commit.signoff` entry in linkgit:gitfaq[7] for more details.