docs: update curl HTTP POST example
docs: update curl HTTP POST example

Must set 'Content-Type: application/json' header due to #1668
mopidy/mopidy,kingosticks/mopidy,jcass77/mopidy,jodal/mopidy,mopidy/mopidy,adamcik/mopidy,jodal/mopidy,jcass77/mopidy,jodal/mopidy,jcass77/mopidy,mopidy/mopidy,kingosticks/mopidy,adamcik/mopidy,kingosticks/mopidy,adamcik/mopidy