thrust::system::cuda::par stream documentation incorrect

(Revised now that I've found the error)

With a `cudaStream_t stream`,

- The [documentation](https://thrust.github.io/doc/group__execution__policies.html#ga64da56b76f487725a99818cd63662dac) says to do `thrust::cuda::par(stream)`.
- The [example](https://github.com/thrust/thrust/blob/8551c97870cd722486ba7834ae9d867f13e299ad/examples/cuda/simple_cuda_streams.cu#L93) says to use `thrust::cuda::par.on(stream)`

The example code works, documentation (the second code block for that method) does not compile.

The compilation error received:

```console
/path/to/sumTest/main.cu(50): error: no instance of function template "thrust::system::cuda::detail::par_t::operator()" matches the argument list
argument types are: (cudaStream_t)
object type is: const thrust::system::cuda::detail::par_t

/path/to/sumTest/main.cu(47): error: no instance of overloaded function "thrust::reduce_by_key" matches the argument list
argument types are: (, thrust::transform_iterator, thrust::counting_iterator, thrust::use_default, thrust::use_default>, thrust::transform_iterator, thrust::counting_iterator, thrust::use_default, thrust::use_default>, thrust::detail::normal_iterator>, thrust::detail::normal_iterator>, thrust::detail::normal_iterator>, thrust::equal_to, thrust::plus)
```

I (quickly) looked around the wiki and did not find any examples that have the same issue, the only mentions of `thrust::cuda::par` are [here](https://github.com/thrust/thrust/wiki/Direct-System-Access#additional-info), but they aren't talking about streams. Anyway, it might be worth somebody else taking a second look :smile: