add comments to explain WithClientID behaviour in the pkce auth example

[dubeyKartikay]

i was building an app using this, and ran into an error while

2026/03/19 17:15:13 oauth2: cannot fetch token: 400 Bad Request
Response: {"error":"invalid_client"}

Just by looking at the example i thought i only had to set the client id here:

	oauth2.SetAuthURLParam("client_id"

but we need to set it using WithClientID during spotifyauth.New

added a comment to tell this to people coming after me