Fix invariant in comment
Fix invariant in comment

`(end >= end)` is trivial, `(end >= start)` is likely intended.
rust-lang-nursery/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang-nursery/regex,rust-lang/regex,rust-lang/regex,rust-lang-nursery/regex