Update the tooltip on "Mark as learned"

[alcpereira]

As an user, I would expect the tooltip to automatically update when clicking on "Mark as learned", either update to "Unmark as learned" (+ changing colour?) or even disappearing from the tooltip.

I'm not familiar with VSCode Extensions API, but I guess this is possible.

I don't know if the present behaviour is a bug or a feature, but as a first time user this has surprised me.