Fixed event key with space

[royeden]

Link to related issue (if applicable)

#2490

Summary of proposed changes

The library switched from event.keyCode to event.key for listeners here: https://github.com/sampotts/plyr/releases/tag/v3.7.1

But the space bar wasn't working because the event.key for spacebar is an empty space string " ". See the mdn reference.

Thank you for such an asewome library! 😄

[sampotts]

Thanks for this!