Fix show-ref usagestring

This describes the abbreviation possibilities for git-show-ref

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-show-ref.c

@@ -3,7 +3,7 @@
 #include "object.h"
 #include "tag.h"
 
-static const char show_ref_usage[] = "git show-ref [-q|--quiet] [--verify] [-h|--head] [-d|--dereference] [-s|--hash] [--tags] [--heads] [--] [pattern*]";
+static const char show_ref_usage[] = "git show-ref [-q|--quiet] [--verify] [-h|--head] [-d|--dereference] [-s|--hash[=<length>]] [--abbrev[=<length>]] [--tags] [--heads] [--] [pattern*]";
 
 static int deref_tags = 0, show_head = 0, tags_only = 0, heads_only = 0,
 	found_match = 0, verify = 0, quiet = 0, hash_only = 0, abbrev = 0;