Update CHANGELOG url (#11070)

_Description of what this PR is changing or adding, and why:_
Update the CHANGELOG url by changing main branch to stable branch
CHANGELOG.md blob url.

_Issues fixed by this PR (if any)_ 
Closes #11067

_PRs or commits this PR depends on (if any):_

## Presubmit checklist

- [ ] This PR is marked as draft with an explanation if not meant to
land until a future stable release.
- [x] This PR doesn’t contain automatically generated corrections
(Grammarly or similar).
- [x] This PR follows the [Google Developer Documentation Style
Guidelines](https://developers.google.com/style) — for example, it
doesn’t use _i.e._ or _e.g._, and it avoids _I_ and _we_ (first person).
- [x] This PR uses [semantic line
breaks](https://github.com/dart-lang/site-shared/blob/main/doc/writing-for-dart-and-flutter-websites.md#semantic-line-breaks)
of 80 characters or fewer.

src/content/release/release-notes/index.md

@@ -138,4 +138,4 @@ For information about bug-fix releases, check out our
 [1.12.13 announcement]: {{site.flutter-medium}}/announcing-flutter-1-12-what-a-year-22c256ba525d
 [1.12.13 release notes and change log]: /release/release-notes/release-notes-1.12.13
 [Archived release notes]: /release/release-notes/release-notes-archive
-[CHANGELOG]: {{site.repo.flutter}}/blob/main/CHANGELOG.md
+[CHANGELOG]: {{site.repo.flutter}}/blob/stable/CHANGELOG.md