chore: add a missing period to rspack description (#86526)

## Summary

Documentation:
- Add a missing period to the `--rspack` option description in the
create-next-app CLI help output.

Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>

Author: Marukome0743

packages/create-next-app/index.ts

@@ -57,7 +57,7 @@ const program = new Command(packageJson.name)
   .option('--src-dir', "Initialize inside a 'src/' directory.")
   .option('--turbopack', 'Enable Turbopack as the bundler.')
   .option('--webpack', 'Enable Webpack as the bundler.')
-  .option('--rspack', 'Enable Rspack as the bundler')
+  .option('--rspack', 'Enable Rspack as the bundler.')
   .option(
     '--import-alias <prefix/*>',
     'Specify import alias to use (default "@/*").'