Fix security-opt docs

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
We talk about conventions for code in the style guide (which I'm working on now and should have a PR by end of week). Currently, the info is in the docs read-me. We'll probably need to go through the docs in toto to make sure we're consistent everywhere.  I'm fine with how this is now. No need to hold up the PR.