Fix error in Planck makefile comment

Fix error in Planck makefile comment

[This comment](https://www.reddit.com/r/olkb/comments/428umx/rgb_underglow/czaivbc) states that because both audio output and RGB support require the user of timer 3, they can't be enabled at the same time. That makes sense, I can see where audio.c uses timer 3. But this comment in the code states that the incompatibility is with MIDI support, which doesn't make sense based on what I see in the code. Please enlighten me if I'm mistaken.