help: Add heading between an unordered and ordered list.

Fixes #35121.

The ordered list in question when converted to the new help center
looked weird when it was following just after an unordered list. We
add a heading in between to make it look nice.

We could not find any other instances of similar pattern occurring
directly in the markdown. The pattern in this case was also happening
b/w two different imported markdown files, we have not checked for the
pattern post-import since that would be too complicated for the purposes
of our issue.

Co-authored-by: Alya Abbott <2090066+alya@users.noreply.github.com>

help/self-hosted-billing.md

@@ -234,6 +234,8 @@ An organization is considered to be a new customer if:
 
 {!manage-billing-intro.md!}
 
+### Manage billing
+
 {start_tabs}
 
 {!self-hosted-billing-log-in-step.md!}