Add Gaussian skewers option into example param file

[jfarr03]

This is just a reminder for me to add the Gaussian skewers option back into an example param.cfg file once PR #36 has been merged.

#Do you want to Gaussian field skewers (as opposed to physical density)? gauss_skewers=true

[damonge]

I've done it in #50 and this will be automatically closed when that gets merged, so no worries!