Update signature of `sample` in tutorial `intro_long`
"```python\n",
"def sample(\n",
" name: str,\n",
- " distribution: pyro.distributions.Distribution,\n",
+ " fn: pyro.distributions.Distribution,\n",
" *,\n",
" obs: typing.Optional[torch.Tensor] = None,\n",
" infer: typing.Optional[dict] = None\n",
