docs: Fix example in Discord oauth cookbook

[gustojs]

Both Discord and @feathers/authentication-oauth expect "scope" key in config file instead of "scopes".

[gustojs]

There's a mistake in my commit title but not in the commit itself. I confirm it has to be "scope", singular. It's plural now.

[gustojs]

I've added another fix in second commit - we need to import oauth, not expressOauth from @feathersjs/authentication-oauth.

[daffl]

Thank you!