fix(examples): correct fastify oauth2 import in examples

[mrbonk97]

This PR fixes incorrect example code by replacing the deprecated
fastify-oauth2 package with the correct scoped package @fastify/oauth2.

The previous example caused runtime errors for users following the documentation.
This change aligns the examples with the current Fastify OAuth2 plugin.

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[gurgunday]

lgtm