Update cliconf-> __init__.py -> get_capabilities() (#79097)

Doc string was incorrect. 
The `get_capabilities` method returns a Python Dict data structure not a JSON string as stated.

lib/ansible/plugins/cliconf/__init__.py

@@ -276,7 +276,7 @@ class CliconfBase(AnsiblePlugin):
                 'diff_replace': [list of supported replace values],
                 'output': [list of supported command output format]
             }
-        :return: capability as json string
+        :return: capability as dict
         """
         result = {}
         result['rpc'] = self.get_base_rpc()