xilem_web: Fix unused_parens lint (linebender#1121)

This is newly triggered in `nightly`, so it breaks building the docs in
CI.

Author: waywardmonkeys

xilem_web/src/message.rs

@@ -21,7 +21,7 @@ pub trait Message: 'static {
     /// Convert `self` into a [`Box<dyn Any>`].
     fn into_any(self: Box<Self>) -> Box<dyn Any>;
     /// Convert `self` into a [`Box<dyn Any>`].
-    fn as_any(&self) -> &(dyn Any);
+    fn as_any(&self) -> &dyn Any;
     /// Gets the debug representation of this message.
     fn dyn_debug(&self) -> &dyn Debug;
 }