Suggestion for "Client Credentials Flow" section in Docs.

[AdriaPadilla]

Hi!
After proposing this library to some python students to learn, I have seen that "Client Credentials Flow" documentation section, is not clear. I think it would be more easy to understand for newbies if:

SPOTIPY_CLIENT_ID = ''
SPOTIPY_CLIENT_SECRET = ''

auth_manager = SpotifyClientCredentials(SPOTIPY_CLIENT_ID,SPOTIPY_CLIENT_SECRET)
sp = spotipy.Spotify(auth_manager=auth_manager)

The current example uses "export" for the credentials, and I think this is confusing.

Thanks!

[stephanebruckert]

Would love a PR for that!