Add missing subcommand when setting example SNS output in the "Getting Started" section

[jordan-wright]

cc: @airbnb/streamalert-maintainers

Background

Reason for the change

The current Getting Started instructions don't mention that you need to add the set command. As it stands, this is the error I received when setting up Streamalert:

(.env) jordan@mac:~/src/aws/streamalert/streamalert$ python manage.py output aws-sns
usage: manage.py output [-h]
                        {set,set-from-file,generate-skeleton,get,list} ...
manage.py output: error: invalid choice: 'aws-sns' (choose from 'set', 'set-from-file', 'generate-skeleton', 'get', 'list')

Changes

• Added subcommand to the python manage.py output instructions.

Testing

Received an error when running the command as prescribed in the documentation. Things seem to work when I added the subcommand.