Broken URL for leaflet.sidepanel plugin

Checklist

• I've searched through the current issues to make sure this change hasn't been suggested already.
• I agree to follow the Code of Conduct that this project adheres to.

Which documents need updating?

https://leafletjs.com/plugins.html#user-interface

Motivation

The original repository link is down because the author's GitHub account is no longer active.

Suggested changes

I suggest updating the plugin link to https://github.com/cyclingbyte/Leaflet.SidePanel or https://www.npmjs.com/package/leaflet.sidepanel