Spec repo does not refer to patch tests in this repo

[cknaap]

The FHIR specification repository mentions the patch tests in https://github.com/HL7/fhir/blob/8faa2c936c1420e8a00b67e3b7b31f4c08ade4c5/tests/readme.txt#L5, but the tests themselves do not live in that directory anymore. I discovered they now live in this repository (https://github.com/FHIR/fhir-test-cases/tree/master/r5/patch).

I'd update the readme in the specification repo to point to their new location.

[grahamegrieve]

sorted in R6 - will take it's time to make it into the repository