improve command formatting

Signed-off-by: Ben Bodenmiller <bbodenmiller@hotmail.com>

docs/insecure.md

@@ -57,9 +57,11 @@ This is more secure than the insecure registry solution.  You must configure eve
 
 1. Generate your own certificate:
 
+```
     mkdir -p certs && openssl req \
       -newkey rsa:4096 -nodes -sha256 -keyout certs/domain.key \
       -x509 -days 365 -out certs/domain.crt
+```
 
 2. Be sure to use the name `myregistrydomain.com` as a CN.