fix docstring of DiffusionCoupling term
fix docstring of DiffusionCoupling term

sfepy/sfepy,vlukes/sfepy,sfepy/sfepy,lokik/sfepy,rc/sfepy,BubuLK/sfepy,lokik/sfepy,vlukes/sfepy,RexFuzzle/sfepy,sfepy/sfepy,RexFuzzle/sfepy,RexFuzzle/sfepy,lokik/sfepy,BubuLK/sfepy,rc/sfepy,RexFuzzle/sfepy,vlukes/sfepy,rc/sfepy,lokik/sfepy,BubuLK/sfepy