unpack-trees.c: align submodule error message to the other error messages

As the place holder in the error message is for multiple submodules,
we don't want to encapsulate the string place holder in single quotes.

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

unpack-trees.c

@@ -167,7 +167,7 @@ void setup_unpack_trees_porcelain(struct unpack_trees_options *opts,
 	msgs[ERROR_WOULD_LOSE_ORPHANED_REMOVED] =
 		_("The following working tree files would be removed by sparse checkout update:\n%s");
 	msgs[ERROR_WOULD_LOSE_SUBMODULE] =
-		_("Submodule '%s' cannot checkout new HEAD");
+		_("Cannot update submodule:\n%s");
 
 	opts->show_all_errors = 1;
 	/* rejected paths may not have a static buffer */