DOCS-13993: [mongocli] fix typo in whitelist describe (#320)

Author: steveren

source/reference/atlas/whitelist-describe.txt

@@ -26,7 +26,7 @@ Syntax
 .. code-block:: text 
    :copyable: false
 
-   mongocli atlas cluster describe <whitelist-entry>
+   mongocli atlas whitelist describe <whitelist-entry>
         [ --output|-o <output-format> ]
         [ --profile|-P <profile-name> ]
         [ --projectId <project-ID> ]