using incorrect erf #357

black_scholes_dpnp.py calls np.erf(...), but dpnp.erf was removed from the top-level namespace.

Fix: import erf from dpnp.scipy.special and
call the bare erf(...).

import dpnp as np
from dpnp.scipy.special import erf
...
d1 = 0.5 + 0.5 * erf(w1)
d2 = 0.5 + 0.5 * erf(w2)