I can't google it for you, but I can point out that you may want to read the linked suggestion yourself and consider reading the whole sentence before jumping down peoples throats.

"It's so nice to run a raid when everyone is up to snuff on their characters and can focus entirely on what THEIR doing."

'3. Remember that they're is a contraction of the words they and are. It can never be used as a modifier, only as a subject (who or what does the action) and verb (the action itself).'

Subject - They, the people doing the DPS.
Verb - Doing.

"focus on what they're doing.*"

That said I don't see the point in jumping on the author consistantly every time there is a mispelling in a blog post. The authors are just human and mistakes and accidents happen.

Also in relation to the article itself, the hardest people to help are usually the ones who won't help themselves. Most people can be encouraged along by a few kind words and a helpful explanation of going wrong. You do get some people who consistantly won't listen or are convinced they're in the right who won't listen but I think those people are a minority and the vast majority of poorer players just needs the right encouragement and patience.