Ignore clippy::type_complexity lint in example

```
warning: very complex type used. Consider factoring parts into `type` definitions
   --> examples/hyper-server.rs:132:15
    |
132 |     Handshake(Pin<Box<dyn Future<Output = io::Result<TlsStream<Async<TcpStream>>>> + Send>>),
    |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(clippy::type_complexity)]` on by default
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#type_complexity
```

examples/hyper-server.rs

@@ -129,6 +129,7 @@ enum SmolStream {
     Tls(TlsStream<Async<TcpStream>>),
 
     /// A TCP connection that is in process of getting secured by TLS.
+    #[allow(clippy::type_complexity)]
     Handshake(Pin<Box<dyn Future<Output = io::Result<TlsStream<Async<TcpStream>>>> + Send>>),
 }