docs: remove comment about credentials flag

The `credentials: true` is always needed since the client is created
with `withCredentials: true`.

README.md

@@ -38,7 +38,7 @@ const httpServer = createServer();
 const io = new Server(httpServer, {
   cors: {
     origin: ["https://admin.socket.io"],
-    credentials: true // needed for cookie-based sticky sessions, else you can ignore this line
+    credentials: true
   }
 });