Fix spelling of a JSON key

[exp625]

The JSON key for the item object in the CurrentlyPlaying struct is now in lowercase. See https://developer.spotify.com/documentation/web-api/reference/#endpoint-get-the-users-currently-playing-track

[strideynet]

Thank you, has all appropriate test data been corrected as well?

[exp625]

Looks like the test data already uses the lowercase spelling for item: https://github.com/zmb3/spotify/blob/9f0fbd9ccd0d25c1cabebd1023a693d8d67845b7/test_data/player_currently_playing.txt#L5

The only occurrence of "Item" for the json key I found was in https://github.com/zmb3/spotify/blob/adbbf425dd35c4dcac21a8ffff2ce8a2d9af800a/player.go#L63

[strideynet]

Awesome, thanks.