[atomics.types.operations] note typo

The note in [atomics.types.operations] p25 (N4861) contains the following code example of the effects of the `compare_exchange_strong` operation:
```
if (memcmp(this, &expected, sizeof(*this)) == 0)
  memcpy(this, &desired, sizeof(*this));
else
  memcpy(expected, this, sizeof(*this));
```
This code is ill-formed, because `expected` in the else-branch is missing a leading `&` operator.