Replace `error!` by `return_error!`
The `error!` macro doesn't seem to exist anymore, so I fixed this!
@@ -518,7 +518,7 @@ macro_rules! return_error (
//FIXME: error rewrite
/// Add an error if the child parser fails
///
-/// While error! does an early return and avoids backtracking,
+/// While `return_error!` does an early return and avoids backtracking,
/// add_return_error! backtracks normally. It just provides more context
/// for an error
///
