Remove .html from URLs to Flarum 2.x docs

[WalsGit]

Hi!

URLs to Flarum 2.x docs for more information result in a 404 page because they have .html like in this example:

cli/boilerplate/stubs/backend/event-listener.php

Line 12 in fc3b076

// See https://docs.flarum.org/2.x/extend/backend-events.html for more information.

here's where are the others to modify:
https://github.com/search?q=repo%3Aflarum%2Fcli+.html+path%3A%2F%5Eboilerplate%5C%2Fstubs%5C%2F%2F&type=code