Fix typo in rustc_lexer docs
Fix typo in rustc_lexer docs

Also add an Oxford comma while we're editing that line.
aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust