CONTRIBUTING.rst needs to be revised to reflect W3C test repository

[hsolbrig]

CONTRIBUTING.rst states:

As of early 2020, the process to generate an EARL report for the official JSON-LD Processor Conformance page is:

    Run the tests on the json-ld-api and json-ld-framing test repos to generate a .jsonld test report:
        python tests/runtests.py ../json-ld-api/tests/ ../json-ld-framing/tests/ -e pyld-earl.jsonld

which sort of implies that we should be using the digitalbazaar tests. We should update this to make it clear that these tests now come from the w3c repositories

[hsolbrig]

Note: I'll submit a pull request for this fix