README incorrectly describes how to launch the application

[mgrider]

It just says to use cargo run, but as near as I can tell, you have to cd into the launchers/native folder, and then execute ./run-native.sh.

I'm pretty new to Bevy, so correct me if I'm wrong!

I also think there's a typo in launchers/native/run-native.sh, in the error message on line 6, it refers to the "iOS launcher directory", and I'm guessing that's copy-pasta.

[simbleau]

Doh! Yes, PRs appreciated if you prefer :)