Music-Reactive Lights won't run because spectrogram moved to ulab.utils

[ggsato]

• List the steps to reproduce the problem below (if possible attach a sketch or copy the sketch code in too): LIST REPRO STEPS BELOW

Follow the guide, then you'll get an import error.

The cause is that spectrogram moved to utils as committed here. So, the fix is simply update the import statement. I will make a PR.