index-pack: fix trivial typo in usage string

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-index-pack.c

@@ -11,7 +11,7 @@
 #include "exec_cmd.h"
 
 static const char index_pack_usage[] =
-"git index-pack [-v] [-o <index-file>] [{ ---keep | --keep=<msg> }] [--strict] { <pack-file> | --stdin [--fix-thin] [<pack-file>] }";
+"git index-pack [-v] [-o <index-file>] [{ --keep | --keep=<msg> }] [--strict] { <pack-file> | --stdin [--fix-thin] [<pack-file>] }";
 
 struct object_entry
 {