update navier_stokes2d.py example for new region syntax
update navier_stokes2d.py example for new region syntax

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