Change hierarchy of security section in README

The security section is currently an h3 'under' the h2 'Getting updates', but isn't related to getting updates.

Change it to an h2 so that it's a sibling of 'Getting updates' rather than a child, thereby starting a new section.

README.md

@@ -78,7 +78,7 @@ To be notified when there’s a new release, you can either:
 
 Find out how to [update with npm](https://frontend.design-system.service.gov.uk/updating-with-npm/).
 
-### Security
+## Security
 
 GDS is an advocate of responsible vulnerability disclosure. If you’ve found a vulnerability, we would like to know so we can fix it.