Update "Learn how to create filters" link on Settings page

Background

The "How to write filters" page has been rewritten and moved to the help center so it can be easily maintained. It's only in English right now, but as I understand, the page on ABP.org is only partially translated in a few languages. Therefore, the fact that it's English only at this time shouldn't be a blocker.

What to do

Update the Learn how to create filters link as specified in eyeo/specs/spec!231.