Fix typo in comment

[lochel]

This commit fixes a typo in api/mraa/gpio.h.

[alext-mkrs]

@lochel, it's definitely kind of an overkill for such a minor change, but if you're willing to steward this one to completion, you'd need to sign the Eclipse ECA (see CONTRIBUTING.md). Otherwise I think we'll introduce a spell checker lint at some point and do a sweep for existing typos, so this one will get fixed eventually.