improve Adding Reports documentation

* `reverse` was used but not imported in the wagtail_hooks.py examples, causing them to be invalid
* I switched the Django `url` calls to use `path` instead since that seems both cleaner and officially recommended, the url function is "likely to be deprecated in a future release" per Django docs and  `path` is starting to replace it in urls.py files now

docs/advanced_topics/adding_reports.rst

@@ -161,7 +161,7 @@ url for the report, you will need to use the ``register_admin_urls`` hook (see :
 
     # <project>/wagtail_hooks.py
 
-    from django.conf.urls import url
+    from django.urls import path, reverse
 
     from wagtail.admin.menu import AdminOnlyMenuItem
     from wagtail.core import hooks
@@ -175,7 +175,7 @@ url for the report, you will need to use the ``register_admin_urls`` hook (see :
     @hooks.register('register_admin_urls')
     def register_unpublished_changes_report_url():
         return [
-            url(r'^reports/unpublished-changes/$', UnpublishedChangesReportView.as_view(), name='unpublished_changes_report'),
+            path('reports/unpublished-changes/', UnpublishedChangesReportView.as_view(), name='unpublished_changes_report'),
         ]
 
 Here, we use the ``AdminOnlyMenuItem`` class to ensure our report icon is only shown to superusers. To make the report visible to all users,
@@ -209,7 +209,7 @@ The full code
 
     # <project>/wagtail_hooks.py
 
-    from django.conf.urls import url
+    from django.urls import path, reverse
 
     from wagtail.admin.menu import AdminOnlyMenuItem
     from wagtail.core import hooks
@@ -223,7 +223,7 @@ The full code
     @hooks.register('register_admin_urls')
     def register_unpublished_changes_report_url():
         return [
-            url(r'^reports/unpublished-changes/$', UnpublishedChangesReportView.as_view(), name='unpublished_changes_report'),
+            path('reports/unpublished-changes/', UnpublishedChangesReportView.as_view(), name='unpublished_changes_report'),
         ]
 
 .. code-block:: html
@@ -255,4 +255,4 @@ The full code
         <td valign="top">
             {{ page.last_published_at }}
         </td>
-    {% endblock %}
+    {% endblock %}