Fix Firefox version badge in readme

[fregante]

It still pointed to Refined GitHub.

Also I inlined the custom cron option. The correct call should have been:

npx ghat fregante/ghatemplates/webext --set 'on.schedule=[{"cron": "41 12 1 * *"}]'

So that future updates won't override it when you call

npx ghat

[notlmn]

Oops: 05783b7#r47397743

[hzoo]

thanks @fregante was going to ping you and say thanks earlier!