fix: Typo in unsupported reason message in content libraries [FC-0114] (openedx#38005)

- Fix a simple typo in an unsupported reason message for blocks with children in content libraries

Author: ChrisChV

cms/djangoapps/modulestore_migrator/tests/test_tasks.py

@@ -257,7 +257,7 @@ def test_migrate_node_with_children_components(self):
                 self.course.id.make_usage_key('library_content', 'test_library_content'),
                 None,
                 'The "library_content" XBlock (ID: "test_library_content") has children, '
-                'so it not supported in content libraries. It has 2 children blocks.',
+                'so it is not supported in content libraries. It has 2 children blocks.',
             ),
         )
         self.assertEqual(len(result.children), 0)
@@ -394,7 +394,7 @@ def test_migrate_component_failure(self):
         self.assertEqual(
             reason,
             'The "library_content" XBlock (ID: "test_library_content") has children,'
-            ' so it not supported in content libraries.',
+            ' so it is not supported in content libraries.',
         )
 
     def test_migrate_component_with_static_content(self):

openedx/core/djangoapps/content_libraries/api/blocks.py

@@ -310,7 +310,8 @@ def validate_can_add_block_to_library(
     if block_class.has_children:
         raise IncompatibleTypesError(
             _(
-                'The "{block_type}" XBlock (ID: "{block_id}") has children, so it not supported in content libraries.'
+                'The "{block_type}" XBlock (ID: "{block_id}") has children,'
+                ' so it is not supported in content libraries.'
             ).format(block_type=block_type, block_id=block_id)
         )
     # Make sure the new ID is not taken already: