tweak SECURITY.md formatting

so it renders well on nokogiri.org

[skip ci]

SECURITY.md

@@ -7,6 +7,7 @@ If you would like to report a vulnerablity or have a security concern regarding
 Your report will be acknowledged within 24 hours, and you'll receive a more detailed response within 72 hours indicating next steps in handling your report.
 
 If you have not received a reply to your submission within 48 hours, there are a few steps you can take:
+
 * Contact the current security coordinator (Mike Dalessio <[email protected]>)
 * Contact the core contributor mailing list ([email protected])
 * Join the [nokogiri-talk group](https://groups.google.com/d/forum/nokogiri-talk)