Remove unneeded bounds on S in FromRequest(Parts) implementations

This doesn't actually allow using non-Send or non-Sync state types, but
it makes the docs simpler.

examples/validator/src/main.rs

@@ -70,7 +70,7 @@ where
     type Rejection = ServerError;
 
     async fn from_request(req: Request<B>, state: &S) -> Result<Self, Self::Rejection> {
-        let Form(value) = Form::<T>::from_request(req, state).await?;
+        let Form(value): Form<T> = req.extract().await?;
         value.validate()?;
         Ok(ValidatedForm(value))
     }