Update slip_transmission_toolbox.rst

changed math to properly reflect vector quantities and (descriptive) labels. I am not sure that I get the GSF formula: why n^T at the end and not just n?
Merge pull request #15 from techfips/patch-1

Thanks. I am going to do it for the resolved shear stress as well and the scheme of slip transmission. For the GSF formula you're totally right. The matlab function is still correct but I will modify the doc.