fix websocket upgrade for nuxt devtools

docs/2.guide/3.going-further/12.websocket-servers.md

@@ -47,7 +47,14 @@ const proxyServer = http.createServer((req, res) => {
 
 // upgrade client connection to websocket if requested
 proxyServer.on('upgrade', (req, socket, head) => {
+  const isSocketIo = req.url.startsWith("/socket.io")
+
+  // choose backend depending on url
+  if (isSocketIo) {
     proxySocketIO.ws(req, socket, head)
+  } else {
+    proxyNuxt.ws(req, socket, head)
+  }
 })
 
 proxyServer.listen(4000, () => console.log("proxy is listening"))