fix link in docstring of ProblemDefinition.update_materials()
fix link in docstring of ProblemDefinition.update_materials()

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