Fix broken link in activitypub.md (#1947)

[tedprodromou]

Updated broken link inside warning. Fixes #1947.

[mjankowski]

Isn't this just an anchor link to a different section of the page? I don't fully understand how its broken as-is...

[tedprodromou]

#1947

It's a subtle bug! As it is, clicking the link in a browser directs to https://docs.joinmastodon.org/#schema (the front page) instead of https://docs.joinmastodon.org/spec/activitypub#schema (anchor link on the same page). You can try it here: https://docs.joinmastodon.org/spec/activitypub/#PropertyValue

This change makes the anchor link go to the expected anchor link instead of the front page.

[trwnh]

I wonder if updating the Hugo version will fix this bug more generally... [](#schema) should go to the current page's fragment, but the currently deployed docs page actually generates <a href="/#schema"> for some reason!

[mjankowski]

I wonder if updating the Hugo version will fix this bug more generally...

Based on that guess, I came up with #1957 which seems to fix this locally for me ... and in theory would fix this more generally if we are doing the same thing elsewhere.