6.0 release notes: fix typo

An unfinished parenthetical.

guides/source/6_0_release_notes.md

@@ -686,7 +686,7 @@ Please refer to the [Changelog][active-storage] for detailed changes.
 
 *   Use the `image_processing` gem for Active Storage variants. This replaces using
     `mini_magick` directly.
-    ([Pull Request](https://github.com/rails/rails/pull/32471)
+    ([Pull Request](https://github.com/rails/rails/pull/32471))
 
 *   Replace existing images instead of adding to them when updating an
     attached model via `update` or `update!` with, say, `@user.update!(images: [ … ])`.