docs(tanstack): update code block for API route example and fix syntax spacing

docs/content/docs/integrations/tanstack.mdx

@@ -12,8 +12,8 @@ Before you start, make sure you have a Better Auth instance configured. If you h
 We need to mount the handler to a TanStack API endpoint.
 Create a new file: `/app/routes/api/auth/$.ts`
 
-```ts
-import { auth } from '@/lib/auth'
+```ts title="routes/api/auth/$.ts"
+import { auth } from '@/lib/auth' // import your auth instance
 import { createAPIFileRoute } from '@tanstack/react-start/api'
 
 export const APIRoute = createAPIFileRoute('/api/auth/$')({
@@ -24,6 +24,4 @@ export const APIRoute = createAPIFileRoute('/api/auth/$')({
     return auth.handler(request)
   },
 })
-```
-
-This will allow you to access use the `getSession` method in all of your routes.
+```