fix warning about fortran_src raised at build

[ndarmage]

Hi,

I noticed the following warning at build:
WARNING: The config value fortran_src' has type list', defaults to `str'.

Please, change the line (setup function in fortran_autodoc module):
app.add_config_value('fortran_src', '.', False)
to:
app.add_config_value('fortran_src', ['.'], False)

.. note:: I cannot commit/push - sorry.

Thanks in advance.
Regards.