getNewAccessTokenUsingRefreshToken(refreshToken, params, callback) doc and signature are slightly misleading

[charlesread]

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Issue

The documentation for this method (https://github.com/fastify/fastify-oauth2/blob/master/README.md?plain=1#L230) gives the impression that refreshToken is a string that is the actual refresh token ("eyJhbGciOiJSUzI1NiI...").

Howerver, refreshToken gets passed to simple-oauth2's .createToken() method, which expects an AccessToken object, itself with a refresh_token property, that can be successfully used.

.getNewAccessTokenUsingRefreshToken({refresh_token: 'eyJhbGciOiJSUzI1NiI...'}) works just fine.

[Eomm]

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