Warning message seems a bit confusing

[Hanno Spreeuw]

I got a bit puzzled by this warning message:

"`apply_operator` is deprecated. Use `apply_operator` instead"

What is meant is probably:

"`_apply_operator` is deprecated. Use `apply_operator` instead"

[David Zwicker]

Good point! This is certainly a typo in the warning. I'll correct it soon. Thanks for reporting!