docs: remove source-branch from slsa verification command (#276)

Author: chgl

README.md

@@ -475,7 +475,6 @@ cosign verify \
 slsa-verifier verify-image \
     --source-uri github.com/miracum/fhir-pseudonymizer \
     --source-tag ${IMAGE_TAG} \
-    --source-branch master \
     "${IMAGE_DIGEST_PINNED}"
 ```