Fix headings levels for the 5.1 release notes

docs/releases/5.1.md

@@ -119,7 +119,7 @@ The `wagtail start` command now supports an optional `--template` argument that
 
 ## Upgrade considerations
 
-## Pillow dependency update
+### Pillow dependency update
 
 Wagtail no longer supports Pillow versions below `9.1.0`.
 
@@ -176,7 +176,7 @@ The undocumented `get_pages_with_direct_explore_permission` and `get_explorable_
 
 The undocumented `users_with_page_permission` function in `wagtail.admin.auth` is also deprecated. It can be replaced with `PagePermissionPolicy().users_with_permission_for_instance(action, page, include_superusers)`.
 
-## `GroupPagePermission` now uses Django's `Permission` model
+### `GroupPagePermission` now uses Django's `Permission` model
 
 The `GroupPagePermission` model that is responsible for assigning page permissions to groups now uses Django's `Permission` model instead of a custom string. This means that the `permission_type` `CharField` has been deprecated and replaced with a `permission` `ForeignKey` to the `Permission` model.
 
@@ -235,7 +235,6 @@ permission.save()
 
 During the deprecation period, the `permission_type` field will still be available on the `GroupPagePermission` model and is used to automatically populate empty `permission` field as part of a system check. The `permission_type` field will be removed in Wagtail 6.0.
 
-
 ### The default ordering of Group Editing Permissions models has changed
 
 The ordering for "Object permissions" and "Other permissions" now follows a predictable order equivalent do Django's default `Model` ordering.
@@ -276,7 +275,6 @@ The global util will be removed in a future release. It is recommended that the
 
 Note: The `data-w-tag-options-value` is a JSON object serialised into string. Django's HTML escaping will handle it automatically when you use the `AdminTagWidget`, but if you are manually writing the attributes, be sure to use quotation marks correctly.
 
-
 ### Image Renditions are now cached by default
 
 Wagtail will try to use the cache called "renditions". If no such cache exists, it will fall back to using the default cache.