[DOC] forward.rst pointing to C API

When rendered, the this snipped (copied at the end and that renders as this) :ref:`mne_make_source_space` points to the C API here: https://mne-tools.github.io/dev/manual/c_reference.html#mne-make-source-space

Creating a surface-based source space
#####################################

The fif format source space files containing the dipole locations
and orientations are created with the utility :ref:`mne_make_source_space`.
This utility is usually invoked by the convenience script :ref:`mne_setup_source_space`,
see :ref:`setting_up_source_space`.

This happens in different places.

[hubertjb]

Just checked the link, and here are all the C API references:

• mne_make_source_space
• mne_setup_source_space
• mne_surf2bem
• mne_setup_forward_model
• mne_prepare_bem_model
• mne_do_forward_solution
• mne_forward_solution
• mne_average_forward_solutions

[massich]

As far as I can tell, this was solved.