builtin/diff.c: fix "git-diff" usage string typo

Remove mistaken right square brackets from "git-diff"
usage string. Make the usage string conform to "git-diff"
documentation (Documentation/git-diff.txt).

Signed-off-by: Shaoxuan Yuan <shaoxuan.yuan02@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/diff.c

@@ -28,9 +28,9 @@ static const char builtin_diff_usage[] =
 "git diff [<options>] [<commit>] [--] [<path>...]\n"
 "   or: git diff [<options>] --cached [--merge-base] [<commit>] [--] [<path>...]\n"
 "   or: git diff [<options>] [--merge-base] <commit> [<commit>...] <commit> [--] [<path>...]\n"
-"   or: git diff [<options>] <commit>...<commit>] [--] [<path>...]\n"
-"   or: git diff [<options>] <blob> <blob>]\n"
-"   or: git diff [<options>] --no-index [--] <path> <path>]\n"
+"   or: git diff [<options>] <commit>...<commit> [--] [<path>...]\n"
+"   or: git diff [<options>] <blob> <blob>\n"
+"   or: git diff [<options>] --no-index [--] <path> <path>\n"
 COMMON_DIFF_OPTIONS_HELP;
 
 static const char *blob_path(struct object_array_entry *entry)