fix iterating over subContent
fix iterating over subContent

when trying to @replace a command group, the following exception was
raised:

    Exception occurred:
      File "/usr/lib/python3.6/site-packages/sphinxarg/ext.py", line 96, in print_action_groups
        for k, v in subContent.items():
    AttributeError: 'list' object has no attribute 'items'

and later

    Exception occurred:
      File "/usr/lib/python3.6/site-packages/sphinxarg/ext.py", line 105, in print_action_groups
        for k, v in map_nested_definitions(subContent):
    ValueError: too many values to unpack (expected 2)

ribozz/sphinx-argparse