Fix document for SELinux permissive mode

If we set 'Permissive' mode for SELinux, SELinux policy is not enforced.
But SELinux is enabled.

See below:
- https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Security-Enhanced_Linux/sect-Security-Enhanced_Linux-Working_with_SELinux-Enabling_and_Disabling_SELinux.html
Merge pull request #13 from iwaim/selinux

Fix document for SELinux permissive mode