Fix A# freq. defined in step2

[shu-13]

Checklists

• [x] I have read the CONTRIBUTING document.
• [x] I have checked to ensure there aren't other open Pull Requests for the same change.

What does this implement/fix? Explain your changes.

Fixed the typo in step2.c and step2.py.

Does this close any currently open issues?

No

How has this been tested?

Cloned this version in the actual RaspberryPiMouse V3 which has Raspbian Buster (2020-02-13-raspbian-buster-full.img) installed.

Any other comments?

Checklistの中の第一項目で、Contributing documentではなくcontributing guidelines の方が、表示されているメッセージと一緒なので、guidelinesの方がいいと思います。