branch: error message deleting a branch in use

Let's update the error message we show when the user tries to delete a
branch which is being used in another worktree, following the guideline
reasoned in 4970bedef2 (branch: update the message to refuse touching a
branch in-use, 2023-07-21).

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/branch.c

@@ -262,7 +262,7 @@ static int delete_branches(int argc, const char **argv, int force, int kinds,
 			const char *path;
 			if ((path = branch_checked_out(name))) {
 				error(_("Cannot delete branch '%s' "
-					"checked out at '%s'"),
+					"used by worktree at '%s'"),
 				      bname.buf, path);
 				ret = 1;
 				continue;