Bad link in README.md to kgx-format.md

[chrisroederucdenver]

Describe the bug
The link to kgx-format.md results in "File not found".
The structure of this graph is expected to conform to the Biolink Model standard, as specified in the KGX format specification.

To Reproduce
Bring up the page from https://github.com/biolink/kgx look for the text above, click the link.

Expected behavior
I'd expect a page to come up that describes the format, instead, I get a 404.

Code snippets
None

Additional context
None