Fix issue with rust's linkchecker and mdbook.

Author: ehuss

src/rust-2018/edition-changes.md

@@ -3,7 +3,6 @@
 The following is a summary of changes that only apply to code compiled with
 the 2018 edition compared to the 2015 edition.
 
-## Rust
 - [Non-lexical lifetimes] (future inclusion planned for 2015 edition)
 - [At most once] `?` macro repetition operator.
 - [Path changes]: