net: fix misleading NamedPipeServer example (#6590)

The previous NamedPipeServer doc example seemed to imply that the return
type of `NamedPipeServer::connect()` was a
`Future<Result<some_kind_of_client>>>`, however, `connect()` returns a
`Future<Result<()>>>`. The following line of code reopening the pipe
would shadow the newly connected pipe immediately, making the following
spawned task pointless. Hopefully these changes make it more clear what
should be happening in the example.

tokio/src/net/windows/named_pipe.rs

@@ -75,7 +75,8 @@ use self::doc::*;
 /// let server = tokio::spawn(async move {
 ///     loop {
 ///         // Wait for a client to connect.
-///         let connected = server.connect().await?;
+///         server.connect().await?;
+///         let connected_client = server;
 ///
 ///         // Construct the next server to be connected before sending the one
 ///         // we already have of onto a task. This ensures that the server