Fix typo in advanced_topics.rst

[davids109]

Hello. fixed typo while was looking at docs for the MIDI controller i'm building.
Best regards.

[ronso0]

Thank you for this PR!
Even though this is just a tiny change we nned zo do the paperwork:

As a first-time contributor we need you to sign the Mixxx Contributor Agreement and comment here when you have done so. It gives us permission to distribute your contribution under the GPL v2 or later license and the Apple Mac App Store. It is also helpful for us to have contact information for contributors in case we may need it in the future.

[ronso0]

Friendly ping @davids109
Pease sign the CLA and we can merge this.
Thank you!

[davids109]

Friendly ping @davids109 Pease sign the CLA and we can merge this. Thank you!

done bro, my bad i didn't see your first reply from 4 weeks ago.

[ronso0]

Thank you 👍