Example needs to be renamed to include simpletest

[kattni]

Example register_struct.py could be renamed to register_struct_simpletest.py to match CircuitPython library standards.

The lines including this file name in the docs/examples.rst file must also be updated to reflect the name change, e.g.:

.. literalinclude:: ../examples/current_file_name.py
    :caption: examples/current_file_name.py
    :linenos:

would be updated to:

.. literalinclude:: ../examples/current_file_name_changed.py
    :caption: examples/current_file_name_changed.py
    :linenos:

[kattni]

Fixed by #30