Donor Metadata Specification link is broken

[cebriggs7135]

From here:
https://github.com/hubmapconsortium/portal-docs/blob/master/metadata.md
The Donor Metadata Specification link is broken:

[mccalluc]

@cebriggs7135 : For links to work on the site, they should be of the form /docs/* with no .md ... that is in tension with the what we'd want to make them work just inside github. We can't have both working at the same time.