docs: fix code block

Author: lalit2005

docs/custom-domains.md

@@ -1,17 +1,31 @@
 You can have the documentation site hosted on a custom domain with help of rewrites
 
 <Callout type='warning'>
-	This feature still has a slight disadvantage - it's not possible to have `customdomain.com/docs/` yet. Instead one can have `customdomain.com/<slug>/docs`
+	This feature still has a slight disadvantage - it's not possible to have **customdomain.com/docs/** yet. Instead one can have **customdomain.com/SLUG/docs**
 </Callout>
 
+Replace all `SLUG`s here with the one you created while creating site in hyperdocs dashboard
+
 We can use any service to rewrite all the routes to our documentation site from the custom domain.
 
 Let's use Vercel here
 
 - Create a new repo
 - Add your custom domain/subdomain to that repo
 - Add a `vercel.json` file with the following content:
-  `json { "$schema": "https://openapi.vercel.sh/vercel.json", "rewrites": [ { "source": "/", "destination": "https://hyperdocs.netlify.app/SLUG" }, { "source": "/:path*", "destination": "https://hyperdocs.netlify.app/:path*" } ] } `
+
+```json
+{
+  "$schema": "https://openapi.vercel.sh/vercel.json",
+  "rewrites": [
+    { "source": "/", "destination": "https://hyperdocs.netlify.app/SLUG" },
+    {
+      "source": "/:path*",
+      "destination": "https://hyperdocs.netlify.app/:path*"
+    }
+  ]
+}
+```
 
 Now you can access the website from your custom domain.