Update "Get number of dimming zones" test functionality in test document

In section https://github.com/rdkcentral/rdkv-halif-test-tvsettings/blob/main/docs/pages/tv-settings_High_Level_Test_Spec.md#testing-scope, there is a test functionality "Get number of dimming zones".
But that doesn't have a definition. On clicking it's not navigating to any section.

Need to provide a definition  section for the test functionality "Get number of dimming zones"