update cassandra.yml.example docs for clarity

Change-Id: I6f0a17aee45bf98e1a39e93ccadb7ee3ef1c3cae
Reviewed-on: https://gerrit.instructure.com/35679
Reviewed-by: Ethan Garofolo <egarofolo@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Tested-by: Ethan Garofolo <egarofolo@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>