Add line to acknowledge mistakes in inclusive language section

Coming out of a discussion in the public speaking workshop @williamchia and I led at Contribute 2019, I propose adding this line to the inclusive language section of our handbook.

Graceful acknowledgement of a mistake is in line with good character values and doesn't leave the person who may have been offended or targeted by non-inclusive language feeling like they caused a "big" problem - or put them on the spot to spend more time and emotional energy to educate and/or reassure their colleague.