Fixes accidental “not” in sentence regarding avoiding deadlock

I came across this sentence in the NSOperation article, I might be wrong by I'm not sure if the "not" in this sentence is intentional.

>“..so it is critical to implement this correctly so as to not avoid deadlock..” 

changed to 

> “..so it is critical to implement this correctly so as to avoid deadlock..”

Great article by the way, clarified a lot for me!