Remove #![feature(core)] from README.md example

[jackc]

When running with latest stable I get this error:

error[E0554]: #[feature] may not be used on the stable release channel --> src/main.rs:1:1 | 1 | #![feature(core)] | ^^^^^^^^^^^^^^^^^

However, it appears that this project works fine on stable. At least I was able to compile and use rustbox successfully without that feature line.

Sorry if I'm missing something obvious -- this is day 2 of my dive into Rust.

[gchp]

Thanks! And apologies for the delay getting to this.