the RE API link from github is broken

[realmarcin]

I was trying to navigate from the github README link here:

These specifications are used by the [Relation Engine API](https://github.com/kbase/relation_engine/blob/develop/spec/relation_engine_server).

This leads to a github page with a 404 error and message:
The 'kbase/relation_engine' repository doesn't contain the 'spec/relation_engine_server' path in 'develop'.

I think the documentation is pretty out of date. The relation engine used to be different repositories and it was majorly reorganized a few times or so. The specs should be under https://github.com/kbase/relation_engine/tree/develop/spec now