message: Fix bucket name in README.md
diff: @@ -130,7 +130,7 @@ recent model.
  ```bash
  export BUCKET_NAME=your_bucket;
  gcloud auth application-default login
  -gsutil ls gs://minigo/models | tail -3
  +gsutil ls gs://$BUCKET_NAME/models | tail -3
  ```
  Which might look like:
  @@ -151,7 +151,7 @@ model to the directory specified by `MINIGO_MODELS`
  ```shell
  MINIGO_MODELS=$HOME/minigo-models
  mkdir -p $MINIGO_MODELS
  -gsutil ls gs://minigo/models | tail -3 | xargs -I{} gsutil cp "{}" $MINIGO_MODELS
  +gsutil ls gs://$BUCKET_NAME/models | tail -3 | xargs -I{} gsutil cp "{}" $MINIGO_MODELS
  ```
  Selfplay
  