Fix RandomNumberGenerator::rand_weighted return type

[luevano]

Fixes wrong return type introduced in #88883:

RandomNumberGenerator::rand_weighted returns int instead of int64_t (the returned value i is defined as int64_t inside the for loop).

Also made minor changes to the documentation example (few typos).

As a side note, can we change the internal RandomPCG::rand_weighted method name to random_weighted (all other methods here are just called random)? The exposed one from RandomNumberGenerator would remain the same rand_weighted.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉