.github/plugins/azure-sdk-dotnet/skills/azure-mgmt-apimanagement-dotnet skill has a broken ref

.github/plugins/azure-sdk-dotnet/skills/azure-mgmt-apimanagement-dotnet/SKILL.md has a broken reference since references/policies.md doesn't exist:

## Reference Files

| File | When to Read |
|------|--------------|
| [references/service-management.md](references/service-management.md) | Service CRUD, SKUs, networking, backup/restore |
| [references/apis-operations.md](references/apis-operations.md) | APIs, operations, schemas, versioning |
| [references/products-subscriptions.md](references/products-subscriptions.md) | Products, subscriptions, access control |
| [references/policies.md](references/policies.md) | Policy XML patterns, scopes, common policies |

(Side-note: I stumbled upon this while playing with https://github.com/rsn491/ctxlint)