Fix broken link to Avro spec

[philrz]

What's Changing

Fixing a broken link to the Avro site.

Why

The CI link checker spotted it when I put up unrelated changes in #5243.

Details

It looks like the Avro site may be mid-overhaul and I suspect they might make more changes before the dust settles. The "specs" that were at the old link are now at a URL of the format https://avro.apache.org/docs/1.12.0/specification/ with a locked-in version string, and current or latest doesn't work when I try it in place of the 1.12.0. Meanwhile they do have a selection called ++version++ in their left-hand nav such that the URL https://avro.apache.org/docs/++version++/specification/ does work and I suspect that's their equivalent of "current" now. But that looks ugly enough that I suspect if we start linking to it, they might well break it again if they find a way to put current or latest there instead.

Meanwhile, our ZNG format doc also happens to link to Avro but in that case it just points to the top-level site, so in the interest of simplicity I've just proposed doing the same here. 🤪