Update spotify-api.d.ts (#178)

This keeps the PlaylistObjectFull/Simplified up to date with the documentation (https://developer.spotify.com/documentation/web-api/reference/object-model/#playlist-object-simplified).

I've also submitted a PR on DefinitelyTyped/DefinitelyTyped#46514

Author: UlysseM

src/typings/spotify-api.d.ts

@@ -961,6 +961,7 @@ declare namespace SpotifyApi {
    */
   interface PlaylistBaseObject {
     collaborative: boolean;
+    description: string | null;
     external_urls: ExternalUrlObject;
     href: string;
     id: string;
@@ -978,7 +979,6 @@ declare namespace SpotifyApi {
    * [](https://developer.spotify.com/web-api/object-model/)
    */
   interface PlaylistObjectFull extends PlaylistBaseObject {
-    description: string;
     followers: FollowersObject;
     tracks: PagingObject<PlaylistTrackObject>;
   }