selinux: fix spelling error

Fix spelling error in selinux module comments that were identified
using the codespell tool.
No functional changes - documentation only.

Signed-off-by: Tanya Agarwal <tanyaagarwal25699@gmail.com>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>