Update apparmor.md

I'm a little confused about which one is better to be used here, a period (.) or a colon (:),  as a command is given below. Or both are OK, and we only have to keep consistency in a single page.

engine/security/apparmor.md

@@ -173,7 +173,7 @@ profile docker-nginx flags=(attach_disconnected,mediate_deleted) {
         -p 80:80 -d --name apparmor-nginx nginx
     ```
 
-4. Exec into the running container
+4. Exec into the running container.
 
     ```bash
     $ docker exec -it apparmor-nginx bash