VQE documentation return value cost function change

[L-git-tech]

In the documentation, subchapter "Define VQE cost function" (http://docs.qulacs.org/en/latest/apply/6.2_vqe.html#Define-VQE-cost-function) the return value of the cost function should be np.real(qulacs_hamiltonian.get_expectation_value(state))
instead of qulacs_hamiltonian.get_expectation_value(state).
This is because of the scipy.optimize.minimize function in http://docs.qulacs.org/en/latest/apply/6.2_vqe.html#Run-VQE, which expects a cost function returning a float number.