grep: Remove unnecessary `String::from`
grep: Remove unnecessary `String::from`

Clippy suggested:
https://travis-ci.org/exercism/rust/builds/456356560
warning: identical conversion
   = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#identity_conversion

petertseng/exercism-rust,petertseng/exercism-rust,exercism/xrust,exercism/xrust,petertseng/exercism-rust