This changed `an String` to `a String`. Very minor change!
The usage of `an String` was introduced in https://github.com/rust-lang/rust/commit/a828e7948069f310dc5b33be8edb65e5e8e0cf9a#diff-b596503c7c33ce457b6d047e351ac12bR423, which changed `an OsString` to `an String`.