Error on generating failure message for assertion 6.4.23

[ricardoas]

Got this error when running the check against a service:

Traceback (most recent call last):
  File "rf_client.py", line 780, in <module>
    main()
  File "rf_client.py", line 774, in main
    rfs_test.run(sut)
  File "/home/ubuntu/redfish/Redfish-Service-Conformance-Check/rfs_test/__init__.py", line 39, in run
    TEST_protocol_details.run(sut, log)
  File "/home/ubuntu/redfish/Redfish-Service-Conformance-Check/rfs_test/TEST_protocol_details.py", line 5046, in run
    assertion_status = Assertion_6_4_23(self, log)
  File "/home/ubuntu/redfish/Redfish-Service-Conformance-Check/rfs_test/TEST_protocol_details.py", line 1439, in Assertion_6_
    log.assertion_log('Unable to verify PATCH for resource readonly property' & (relative_uris[relative_uri]))
TypeError: unsupported operand type(s) for &: 'str' and 'str'

It seems to me the message generation is wrong... I'll submit a PR fixing it.