Update Convex example not to ignore generated files (#38931)

Ignoring files in `convex/_generated/` was a mistake: it's not the currently recommended workflow. Developers should be able to deploy just their frontend without deploying the most recent backend functions at the same time (which would regenerate these files). With this change, a default deploy to Vercel will no longer fail.

## Documentation / Examples

- [x] Make sure the linting passes by running `pnpm lint`
- [x] The examples guidelines are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing.md#adding-examples)

examples/convex/.gitignore

@@ -33,8 +33,5 @@ yarn-error.log*
 # vercel
 .vercel
 
-# convex
-convex/_generated
-
 # typescript
 *.tsbuildinfo