Change fn args for Curand.PRNG

In **def** monte_carlo_pricer(paths, dt, interest, volatility), the call to the PRNG random number generator did not accept the second _stream=stream_ argument. After removing it, the code ran correctly:
old code:
prng = curand.PRNG(curand.PRNG.MRG32K3A, stream=stream)
new code:
prng = curand.PRNG(curand.PRNG.MRG32K3A)