fix formatting/spelling in code block in Controlling a DC Motor with …

[AdamRaichu]

…Motor Shield Rev3

What This PR Changes

There was inconsistent formatting on the page here. I fixed it up a bit and used the correct spelling of brake (as in what is used to stop a motor; the previous spelling was break as in to damage).

Contribution Guidelines

• [x] I confirm that I have read the contribution guidelines and comply with them.

[CLAassistant]

All committers have signed the CLA.