Fix subclassing note comments in AFHTTPRequestOperationManager.h

Right now, both of these lines suggest `AFHTTPSessionManager`. It looks like that was a mistake from reversing the order of the suggestions from `AFHTTPSessionManager.h`, but I think the original wording better suggests that `AFHTTPSessionManager` is preferred if you don't need to support iOS 6 or OS X 10.8. If that's not desired, then you could discard these changes and instead change `AFHTTPSessionManager` in the first line to `AFHTTPRequestOperationManager`.