docs: Update README.md with revised information (#207)

Author: rojandahal

README.md

@@ -43,7 +43,7 @@ fastify.get('/login/facebook/callback', async function (request, reply) {
   console.log(token.access_token)
 
   // if later you need to refresh the token you can use
-  // const { token: newToken } = await this.getNewAccessTokenUsingRefreshToken(token.refresh_token)
+  // const { token: newToken } = await this.getNewAccessTokenUsingRefreshToken(token)
 
   reply.send({ access_token: token.access_token })
 })
@@ -227,7 +227,13 @@ This fastify plugin adds 3 utility decorators to your fastify instance using the
   - `refresh_token` (optional, only if the `offline scope` was originally requested, as seen in the callbackUriParams example)
   - `token_type` (generally `'bearer'`)
   - `expires_in` (number of seconds for the token to expire, e.g. `240000`)
-- `getNewAccessTokenUsingRefreshToken(refreshToken, params, callback)`: A function that takes a refresh token and retrieves a new `AccessToken` object. This is generally useful with background processing workers to re-issue a new token when the original token has expired. The `params` argument is optional and it is an object that can be used to pass in extra parameters to the refresh request (e.g. a stricter set of scopes). If the callback is not passed this function will return a promise. The object resulting from the callback call or the promise resolution is a new `AccessToken` object (see above).
+- `getNewAccessTokenUsingRefreshToken(Token, params, callback)`: A function that takes a `AccessToken`-Object as `Token` and retrieves a new `AccessToken`-Object. This is generally useful with background processing workers to re-issue a new AccessToken when the previous AccessToken has expired. The `params` argument is optional and it is an object that can be used to pass in additional parameters to the refresh request (e.g. a stricter set of scopes). If the callback is not passed this function will return a Promise. The object resulting from the callback call or the resolved Promise is a new `AccessToken` object (see above). Example of how you would use it for `name:googleOAuth2`:
+```js
+fastify.googleOAuth2.getNewAccessTokenUsingRefreshToken(currentAccessToken, (err, newAccessToken) => {
+   // Handle the new accessToken
+});
+```
+
 - `generateAuthorizationUri(requestObject)`: A function that returns the authorization uri. This is generally useful when you want to handle the redirect yourself in a specific route. The `requestObject` argument passes the request object to the `generateStateFunction`). You **do not** need to declare a `startRedirectPath` if you use this approach. Example of how you would use it:
 
 ```js