"git gud explain": Do not accept user input other than ENTER

Go to `util/level_builder.py` in the function `BasicLevel.explain()` to fix this.

Basically `git gud explain` waits for the user to hit enter, but they can also type anything they want because we use `input` and not something more advanced. This could mess up input or simply make it more confusing for a user, with them possibly thinking they're typing into a command line.