Documentation/config: fix inconsistent label on gc.*.reflogExpireUnreachable

Change <ref> to <pattern> in the description of
gc.*.reflogExpireUnreachable, since that is what the text refers to.
Signed-off-by: NAndreas Schwab <schwab@linux-m68k.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1256,7 +1256,7 @@ gc.<pattern>.reflogExpire::
 	the refs that match the <pattern>.
 
 gc.reflogExpireUnreachable::
-gc.<ref>.reflogExpireUnreachable::
+gc.<pattern>.reflogExpireUnreachable::
 	'git reflog expire' removes reflog entries older than
 	this time and are not reachable from the current tip;
 	defaults to 30 days.  With "<pattern>" (e.g. "refs/stash")