IDL Contribution function comparisons need to be multiplied by 0.83

[wtbarnes]

The documentation build automatically computes some comparisons with some contribution functions computed by IDL: https://fiasco.readthedocs.io/en/latest/generated/gallery/idl_comparisons/goft_comparison.html#. However, the diffs now appear to be signficantly larger than before. I am pretty sure this is due to the exclusion of the factor of $n_H/n_e\approx0.83$ in the contribution function as of #236.