submodule--helper: fix usage string for relative-path

Internally we call the underscore version of relative_path, but externally
we present an API with no underscores.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/submodule--helper.c

@@ -892,7 +892,7 @@ static int resolve_relative_path(int argc, const char **argv, const char *prefix
 {
 	struct strbuf sb = STRBUF_INIT;
 	if (argc != 3)
-		die("submodule--helper relative_path takes exactly 2 arguments, got %d", argc);
+		die("submodule--helper relative-path takes exactly 2 arguments, got %d", argc);
 
 	printf("%s", relative_path(argv[1], argv[2], &sb));
 	strbuf_release(&sb);