Update link for v0.15.0 diff in changelog (#4596)

Follow up to https://github.com/alcionai/corso/pull/4590 to add a missing link.

---

#### Does this PR need a docs update or release note?

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [ ] ⛔ No

#### Type of change

<!--- Please check the type of change your PR introduces: --->
- [ ] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup

#### Issue(s)

<!-- Can reference multiple issues. Use one of the following "magic words" - "closes, fixes" to auto-close the Github issue. -->
* #<issue>

#### Test Plan

<!-- How will this be tested prior to merging.-->
- [ ] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

CHANGELOG.md

@@ -418,7 +418,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Miscellaneous
   - Optional usage statistics reporting ([RM-35](https://github.com/alcionai/corso-roadmap/issues/35))
 
-[Unreleased]: https://github.com/alcionai/corso/compare/v0.14.0...HEAD
+[Unreleased]: https://github.com/alcionai/corso/compare/v0.15.0...HEAD
+[v0.15.0]: https://github.com/alcionai/corso/compare/v0.14.0...v0.15.0
 [v0.14.0]: https://github.com/alcionai/corso/compare/v0.13.0...v0.14.0
 [v0.13.0]: https://github.com/alcionai/corso/compare/v0.12.0...v0.13.0
 [v0.12.0]: https://github.com/alcionai/corso/compare/v0.11.1...v0.12.0