docs: Update README.md with revised information

[rojandahal]

The documentation (https://github.com/fastify/fastify-oauth2/blob/master/README.md?plain=1#L230) explained that the refresh token required is actual refresh token.

But the refresh token passed to simple-oauth2 's .createToken() method expects the Access Token object with refresh token itself.

So the documentation is edited to explain that the .getNewAccessTokenUsingRefreshToken() method required AccessToken object which is of format {refresh_token: 'eyJhbGciOiJSUzI1NiI...'})

Checklist

• run npm run test and npm run benchmark
• 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

[Uzlopak]

Does not seem right

[Uzlopak]

LGTM. :)

[mcollina]

lgtm

[mcollina]

thanks!