gnmi-1.18 inconsistency between README and the code

[LimeHat]

gnmi-1.18 readme lists two test cases that ask to validate ON_CHANGE telemetry only:

https://github.com/openconfig/featureprofiles/blob/afe4b12d8f80e5fb4c847a3d72a17217eb7468fa/feature/platform/fabric/otg_tests/sampled_backplane_capacity_counters_test/README.md?plain=1#L21-L25

https://github.com/openconfig/featureprofiles/blob/afe4b12d8f80e5fb4c847a3d72a17217eb7468fa/feature/platform/fabric/otg_tests/sampled_backplane_capacity_counters_test/README.md?plain=1#L39-L44

In the code, however, there's also a test that validates sampled telemetry as well:

https://github.com/openconfig/featureprofiles/blob/afe4b12d8f80e5fb4c847a3d72a17217eb7468fa/feature/platform/fabric/otg_tests/sampled_backplane_capacity_counters_test/sampled_backplane_capacity_counters_test.go#L53

README or the code needs to be updated to match

cc @cfernanz @dplore