Fix an error in the command about Hierarchical Neural Story Generation (#1099)

Summary:
When I try to reproduce the experiment in  _Hierarchical Neural Story Generation_, I found the command about generation cannot be executed.

It said that **fairseq-generate: error: unrecognized arguments: --sampling-temperature 0.8**
In the document, I find:
```
--temperature   temperature for generation
Default: 1.0
```
And I don't find a parameter named `--sampling-temperature`, so I think the parameter `--sampling-temperature` should be changed to `--temperature`
Pull Request resolved: https://github.com/pytorch/fairseq/pull/1099

Differential Revision: D17163065

Pulled By: myleott

fbshipit-source-id: 25c430eeee4703f8ec30353825ffec4bb973da0d

examples/stories/README.md

@@ -52,7 +52,7 @@ fairseq-train data-bin/writingPrompts -a fconv_self_att_wp --lr 0.25 --clip-norm
 # Generate:
 # Note: to load the pretrained model at generation time, you need to pass in a model-override argument to communicate to the fusion model at generation time where you have placed the pretrained checkpoint. By default, it will load the exact path of the fusion model's pretrained model from training time. You should use model-override if you have moved the pretrained model (or are using our provided models). If you are generating from a non-fusion model, the model-override argument is not necessary.
 
-fairseq-generate data-bin/writingPrompts --path /path/to/trained/model/checkpoint_best.pt --batch-size 32 --beam 1 --sampling --sampling-topk 10 --sampling-temperature 0.8 --nbest 1 --model-overrides "{'pretrained_checkpoint':'/path/to/pretrained/model/checkpoint'}"
+fairseq-generate data-bin/writingPrompts --path /path/to/trained/model/checkpoint_best.pt --batch-size 32 --beam 1 --sampling --sampling-topk 10 --temperature 0.8 --nbest 1 --model-overrides "{'pretrained_checkpoint':'/path/to/pretrained/model/checkpoint'}"
 ```
 
 ## Citation