Fix link to developer license from Dependency Proxy guide

What does this Merge Request do and why?

This MR fixes the link in the Dependency Proxy guide to the Ultimate license section in the handbook. The section title was updated in gitlab-com/www-gitlab-com@e5009682 thus changing the permalink as well.

Clicking the current link right now that not lead to any specific section in the page. This MR updates the link so once the link is clicked, the proper section is focused on.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.