fix docstring of Field.get_mapping()
fix docstring of Field.get_mapping()

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