branch: error message checking out a branch in use

Let's update the error message we show when the user tries to check out
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>

branch.c

@@ -838,7 +838,7 @@ void die_if_checked_out(const char *branch, int ignore_current_worktree)
 
 		if (is_shared_symref(worktrees[i], "HEAD", branch)) {
 			skip_prefix(branch, "refs/heads/", &branch);
-			die(_("'%s' is already checked out at '%s'"),
+			die(_("'%s' is already used by worktree at '%s'"),
 				branch, worktrees[i]->path);
 		}
 	}