submodule--helper: fix BUG message in ensure_core_worktree

74d4731 (submodule--helper: replace connect-gitdir-workingtree by
ensure-core-worktree, 2018-08-13) missed to update the BUG message.
Fix it.

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

builtin/submodule--helper.c

@@ -2045,7 +2045,7 @@ static int ensure_core_worktree(int argc, const char **argv, const char *prefix)
 	struct repository subrepo;
 
 	if (argc != 2)
-		BUG("submodule--helper connect-gitdir-workingtree <name> <path>");
+		BUG("submodule--helper ensure-core-worktree <path>");
 
 	path = argv[1];