Fix EntityStub.exists()

Correcting C++ API method name.
Ref: This API defined in SRCROOT/ecell/libemc/Simulator.hpp

Sample code:
  ecell3-session>>> loadModel('model.eml')
  <model.eml, t=0>>> s = createEntityStub('Variable:/:S')
  <model.eml, t=0>>> s.exists() # formerly failed