fix typo in docstring for GP.opimize()
fix typo in docstring for GP.opimize()

SheffieldML/GPy,SheffieldML/GPy,SheffieldML/GPy,esiivola/GPYgradients,esiivola/GPYgradients,esiivola/GPYgradients,esiivola/GPYgradients,SheffieldML/GPy