Update MIME type for Rust to text/rust.
Update MIME type for Rust to text/rust.

This is what Rust is registered as in shared-mime-info:
https://bugs.freedesktop.org/show_bug.cgi?id=90487

sol/pygments,sol/pygments,sol/pygments,sol/pygments,sol/pygments,sol/pygments,sol/pygments,sol/pygments,sol/pygments,sol/pygments,sol/pygments,sol/pygments,sol/pygments,sol/pygments