Should a toggle button show its current state or the state to which it will change?
(ux.stackexchange.com)
This would be correct if the post was about switches (which TBF many people made that mistake, including me initially), but it's about buttons - buttons should show what action will result from clicking on them. e.g. "Cancel" on a button which is going to cancel your process. For a play/pause button it should show play if you're paused (if I click on this it will start playing) and pause if you're playing (if I click on this it will pause).