fix outdated docs for ParseError (servo#1074)

The referenced unused variant seems to have been removed in commit
52d736e with the move to `#[non_exhaustive]`.

Author: whispersofthedawn

url/src/parser.rs

@@ -52,7 +52,7 @@ macro_rules! simple_enum_error {
         /// Errors that can occur during parsing.
         ///
         /// This may be extended in the future so exhaustive matching is
-        /// discouraged with an unused variant.
+        /// forbidden.
         #[derive(PartialEq, Eq, Clone, Copy, Debug)]
         #[non_exhaustive]
         pub enum ParseError {