Add missing "short" alternative to --date in rev-list-options.txt

Signed-off-by: Heikki Orsila <heikki.orsila@iki.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/rev-list-options.txt b/Documentation/rev-list-options.txt
index 2648a55..9cd6771 100644
--- a/Documentation/rev-list-options.txt
+++ b/Documentation/rev-list-options.txt
@@ -13,7 +13,7 @@
 
 	Synonym for `--date=relative`.
 
---date={relative,local,default,iso,rfc}::
+--date={relative,local,default,iso,rfc,short}::
 
 	Only takes effect for dates shown in human-readable format, such
 	as when using "--pretty".