Fix reference to np.random.poisson (#4452)
Fix reference to np.random.poisson (#4452)

Fixes mypy's complaint:

```
$ ./check/mypy 
cirq-core/cirq/contrib/acquaintance/gates_test.py:20: error: Module 'numpy.random' has no attribute 'poisson'
Found 1 error in 1 file (checked 925 source files)
```

See the code snippet in [this document](https://numpy.org/doc/stable/reference/random/index.html#random-quick-start) for background on this change.
quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq