billing: fix 404 (docker#22638)

## Description
- Found a 404 during triage, added redirect

## Related issues or tickets

https://docker.atlassian.net/browse/ENGDOCS-2618?atlOrigin=eyJpIjoiZmVkNzliNzdhZDI2NDk0NWFlNmVmOWU2ZDJmOTdjMjciLCJwIjoiaiJ9

## Reviews
- [ ] Technical review
- [ ] Editorial review
- [ ] Product review

Author: sarahsanders-docker

content/manuals/subscription/change.md

@@ -12,6 +12,7 @@ aliases:
 - /docker-hub/cancel-downgrade/
 - /docker-hub/billing/downgrade/
 - /billing/scout-billing/
+- /billing/subscription-management/
 weight: 30
 ---