Fix incorrect port number in "Building Your First Activity in Discord" guide (#7750)

* Update building-an-activity.mdx

* Update port numbers

Author: abedef

docs/activities/building-an-activity.mdx

@@ -88,7 +88,7 @@ npm install
 npm run dev
 ```
 
-If you visit http://localhost:3000/ you should see a vanilla JS frontend template running with [Vite](https://vitejs.dev/).
+If you visit http://localhost:5173/ you should see a vanilla JS frontend template running with [Vite](https://vitejs.dev/).
 
 While it's not much at the moment, in the following steps we'll connect it to the backend services, make it runnable in Discord, and power it up by populating it with data we pull from Discord APIs.
 
@@ -278,7 +278,7 @@ Your app should start and you should see output similar to the following:
 ```
 VITE v5.0.12  ready in 100 ms
 
-➜  Local:   http://localhost:3000/
+➜  Local:   http://localhost:5173/
 ➜  Network: use --host to expose
 ➜  press h + enter to show help
 ```
@@ -289,10 +289,10 @@ We'll use the Local URL as our publicly-accessible URL in the next step.
 
 Next, we'll need to set up the public endpoint that serves the Activity's frontend. To do that, we'll create a tunnel with a reverse proxy. While we'll be using [`cloudflared`](https://developers.cloudflare.com/cloudflare-one/connections/connect-networks/) in this guide, you can use [ngrok](https://ngrok.com/docs) or another reverse proxy solution if you prefer.
 
-While your app is still running, open another terminal window and start a network tunnel that listens to the port from the last step (in this case, port `3000`):
+While your app is still running, open another terminal window and start a network tunnel that listens to the port from the last step (in this case, port `5173`):
 
 ```
-cloudflared tunnel --url http://localhost:3000
+cloudflared tunnel --url http://localhost:5173
 ```
 
 When you run `cloudflared`, the tunnel will generate a public URL and you'll see output similar to the following: