📝 Remove duplicate pnpm in Vercel deploy docs (#1261)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Refactor**
- Simplified the build process for Vercel deployment by removing a
redundant command.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

apps/docs/self-hosting/deploy/vercel.mdx

@@ -60,7 +60,7 @@ If you deploy on a Vercel Hobby plan, you will need to reduce the `maxDuration`
 5. Change the build command to:
 
    ```sh
-   cd ../.. && pnpm pnpm turbo build --filter=viewer... && pnpm db:migrate
+   cd ../.. && pnpm turbo build --filter=viewer... && pnpm db:migrate
    ```
 
 6. Add the required environment variables ([Check out the configuration guide](/self-hosting/configuration))