Set the bin_name for the CLI to rubyfmt

[reese]

This is a mega nit but when you run rubyfmt -h, the usage instructions says Usage: rubyfmt-main instead of rubyfmt because clap defaults to the crate name (and we have the binary in rubyfmt-main and the library in rubyfmt). This sets the CLI's bin_name to rubyfmt so that it now says

% cargo run -- -h
Rubyfmt CLI

Usage: rubyfmt [OPTIONS] [include-paths]...