Gene example in docs is a broken link

[jsstevenson]

https://va-spec.ga4gh.org/en/1.0/core-information-model/domain-entities.html#gene -- contains a link to https://github.com/ga4gh/va-spec/blob/1.0.1/examples/mappable-concept-gene.yaml (returns a 404)

I think there are other ways of providing reference to source (eg embedding text direclty on the page) that will cause build errors or at least warnings if the links break