Bug in vqe_run when disp = True

[diogo-cruz]

The documentation of vqe_run in the VQE class indicates that we should use the argument disp = True if we want the expectation and parameters to be printed at each iteration.
However, in line 123 of vqe.py:

self._disp_fun = disp if disp is not None else lambda x: None

So we actually have to use disp = print for it to work as intended.