Fix #854: change NoiseChannel.__str__ to use correct method

An inspection of the code of class `NoiseChannel` in
`qsimcirq_tests/qsimcirq_tests.py` suggests that the most likely cause
of error #854 is that the call to the `_ops` method should be instead to
`_prob_op_pairs`, like this:

```python
def __str__(self):
    return f"NoiseChannel({self._prob_op_pairs})"
```

Author: mhucka

qsimcirq_tests/qsimcirq_test.py

@@ -1045,7 +1045,7 @@ def steps(self):
         return [m for _, m in self._prob_op_pairs]
 
     def __str__(self):
-        return f"NoiseChannel({self._ops})"
+        return f"NoiseChannel({self._prob_op_pairs})"
 
     def __repr__(self):
         return str(self)