MINOR: Security doc fixes
MINOR: Security doc fixes

Simple fixes that have tripped users.

Author: Ismael Juma <ismael@juma.me.uk>

Reviewers: Ewen Cheslack-Postava <ewen@confluent.io>

Closes #745 from ijuma/security-doc-improvements

(cherry picked from commit 36f5c46a5cf510001a4990db6199beb37f215007)
Signed-off-by: Ewen Cheslack-Postava <b1c1d8736f20db3fb6c1c66bb1455ed43909f0d8@ewencp.org>

racker/kafka,racker/kafka,racker/kafka,racker/kafka