message: doc: fix link to regex-syntax
  Fixes
diff: @@ -287,7 +287,7 @@ regexes.
  The syntax supported in this crate is documented below.
  Note that the regular expression parser and abstract syntax are exposed in
  -a separate crate, [`regex-syntax`](../regex_syntax/index.html).
  +a separate crate, [`regex-syntax`](https://docs.rs/regex-syntax).
  ## Matching one character
  