Fix APIHelper class missing in Sphinx docs

After moving APIHelper to its own file _helper.py in #137, the class was not
found by Sphinx autodoc anymore, thus missing in the docs and we didn't catch
the warning at build time.

doc/source/synadm.module.cli.rst

@@ -15,7 +15,7 @@ package that is worth mentioning and generating documentation for, is the
 backend code in :mod:`synadm.api`:
 
 
-.. automodule:: synadm.cli
+.. automodule:: synadm.cli._helper
    :members: APIHelper
    :undoc-members:
    :show-inheritance: