docs(echoreadall): fix function echoReadAll comment (#881)

As of Go 1.16, ioutil.ReadAll was deprecated.

examples/autobahn/server.go

@@ -84,7 +84,7 @@ func echoCopyFull(w http.ResponseWriter, r *http.Request) {
 }
 
 // echoReadAll echoes messages from the client by reading the entire message
-// with ioutil.ReadAll.
+// with io.ReadAll.
 func echoReadAll(w http.ResponseWriter, r *http.Request, writeMessage, writePrepared bool) {
 	conn, err := upgrader.Upgrade(w, r, nil)
 	if err != nil {