Minor revision to examples of sources for commit messages

Our ticketing and support systems are becoming more complicated across
GDS, hopefully this is a minor clarification that avoids a double take
from folks not using zendesk

source/standards/pull-requests.html.md.erb

@@ -35,7 +35,7 @@ You should make sure that:
 
 - your local repository has the most recent changes on the target branch
 - the title and description of the PR are clear and accurately represent your changes
-- the title and description reference the source of the change, which could be a Zendesk ticket or a Jira card
+- the title and description reference the source of the change, which could be a support ticket (Zendesk, or Service Now) or a Jira card
 - the description contains links to any related PRs
 - any screenshots have text descriptions for accessibility
 - any contentious changes or side effects have clear descriptions of the pros and cons