Remove .html from URLs to Flarum 2.x docs

Hi!

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

https://github.com/flarum/cli/blob/fc3b07665c0ae25d46056fafeaf4e4e853d359c2/boilerplate/stubs/backend/event-listener.php#L12

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