Update the link to Axon Framework in `prior-art.md`

[alexander-yevsyukov]

It's in this block:

In Spine Event Engine we combined all of our experience and observations of the best-breed market
products and solutions like [Axon](http://www.axonframework.org/), [Spring](https://spring.io/), [Kurrent](https://www.kurrent.io/) 
(formerly Event Store), [InfluxDB](https://influxdata.com/), [Apache Zest](https://zest.apache.org/) and many others. 
Spine has yet to find its own niche.

The site at http://www.axonframework.org/ does not seem to be available any longer.

The good new address for the framework is this: https://www.axoniq.io/axon-framework

We need to update the link because our link checker for spine.io times out on this link.