Fix security-opt docs

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
@SvenDowideit I will be the captain obvious here and state that this clashes with comments. (no .. Sherlock, I know).

I'm +1000 for using anything-but an already existing symbol. 

Suggestion for a replacement: `$!` 

If however `#` is like a universal standard, and if it's not done already, perhaps guide-for-the-guides should state this fact and confused eejits like me should be reminded to read that ;-)