Update to ntlk download instructions
This matches what's done in Dockerfile, and seems to be more robust.
@@ -37,7 +37,7 @@ Some requirements may only be needed for specific configurations. If you have tr
You will also need to install dependencies for `nltk` if you do not already have them:
```
-python -m nltk.downloader -d perluniprops nonbreaking_prefixes punkt
+nltk.downloader -d ./nltk_data perluniprops nonbreaking_prefixes punkt
```
