Small omission on code flow auth guide

[riokou]

https://developer.spotify.com/documentation/general/guides/authorization/code-flow/

In the section about requesting an access token, the guide does not make it clear that the "Authorization" header is not required if implementing the PKCE extension. Clarifying this is important as the main point of PKCE is that it does not require authorizing with a client secret.

The guide does make it clear that this header is not required in the section about requesting a refreshed Access Token.