fix comment in get_xi_tensor()
fix comment in get_xi_tensor()

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