feat: small improvements to pb-rs arg parsing

[FauxFaux]

Two small improvements to the arg parser in pb-rs:

• spell hashbrown correctly, to save me googling what hashrown is and getting really confused. Technically this is a breaking change.
• Mark input as required, so pb-rs with no args is a bit more helpful:

Before:

pb-rs fatal error No .proto file provided

After:

error: The following required arguments were not provided:
    <INPUT>...

USAGE:
    pb-rs [FLAGS] [OPTIONS] <INPUT>...

For more information try --help

It's not ideal, but it's a bit more comforting to me, who is used to clap output?