Fix Timebox link on Figma Plugin Page

[akigugale]

Reference Screenshot of page: https://www.figma.com/community/plugin/820311256083321341/Timer

The link for timebox has a period in the end. And so it gives the following error

[killnicole]

Thanks @akigugale for reporting! @lennet or @JannesPeters Would you mind publishing the description without a full stop at the end? Thank you!