Launcher: Update message that displays when installing a custom apworld for a game in main

[ScipioWright]

What is this fixing or adding?

When double-clicking a beta Tunic apworld, I got the above message. I did not have the regular Launcher open, so I could see this being confusing for some users. So, this PR updates the message that displays to hopefully be a bit clearer.

How was this tested?

Reading