Add Labels to Sliders in Chapter 20 for clarity

I see it's handled later in Chapter 21 when it is stylized but I thought it'd be handy to add a label to the two sliders so the tutorial learner knows which slider affects music and sfx. I created a Pull Request from a Fork of the code. Please let me know I'm not properly introducing these issues. 

https://github.com/MonoGame/docs.monogame.github.io/pull/216