Fix documentation highlighting in ty::BorrowKind
Fix documentation highlighting in ty::BorrowKind

Previously it looked a little odd: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.BorrowKind.html#variant.UniqueImmBorrow

aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust