Update model download instructions

Some versions of tar require the "j" flag to decompress

Author: keithito

README.md

@@ -36,7 +36,7 @@ pip install -r requirements.txt
 
 1. **Download and unpack a model**:
    ```
-   curl http://data.keithito.com/data/speech/tacotron-20170720.tar.bz2 | tar x -C /tmp
+   curl http://data.keithito.com/data/speech/tacotron-20170720.tar.bz2 | tar xj -C /tmp
    ```
 
 2. **Run the demo server**: