Fix misspelled word unumte -> unmute

[vchelaru]

docs.monogame.github.io/articles/tutorials/building_2d_games/15_audio_controller/index.md

Line 150 in f52c573

Running the game now will produce the same result as the previous chapter, only now the lifetime of sound effects and the state management of audio is done through the new audio controller. You can also mute and unumte the audio with the M key and increase and decrease the volume using the + and - keys.

Should say "unmute" instead of "unumte"