Update example code to run inside `main`

[keoghpe]

Small docs change - I'm new to this crate and found the current docs page confusing as it calls init outside a main function. Hopefully this makes it clearer!

Thanks for creating a cool crate ❤️

[epage]

I'm mixed on this. This makes the intent even more explicit. On the other hand, its not very idiomatic these days to have main in doc tests.