[DOC]: Fix `cub::DeviceAdjacentDifference` docs to require const references

### Is this a duplicate?

- [X] I confirmed there appear to be no [duplicate issues](https://github.com/NVIDIA/cccl/issues) for this bug and that I agree to the [Code of Conduct](CODE_OF_CONDUCT.md)

### Is this for new documentation, or an update to existing docs?

Update

### Describe the incorrect/future/missing documentation

Existing docs show the following example:
```cpp
struct CustomDifference
{
template
__device__ DataType operator()(DataType &lhs, DataType &rhs)
{
return lhs - rhs;
}
};
```
It should be `const DataType &lhs` instead. As part of the sphinx migration, we should cite the code that we compile and verify.

### If this is a correction, please provide a link to the incorrect documentation. If this is a new documentation request, please link to where you have looked.

_No response_