CohFrict doc

I'm correcting a typo here. Maybe someone more knowledgeable than me in CohFrict model can check and take the opportunity to mention in the Law2 doc the possibility for the torque-induced loss of cohesive nature. Which seems to exist in source code, but not in the doc, e.g. If the maximum tensile or maximum shear force is reached and CohFrictPhys::fragile =True (default), the cohesive link is broken