fix migration code samples in docs

Author: bckohan

doc/source/changelog.rst

@@ -46,7 +46,7 @@ Migration from 1.x -> 2.x
 
 * Imports of ``EnumChoiceField`` for django forms has been changed:
 
-    ..code-block: python
+    .. code-block:: python
 
         # 1.x way
         from django_enum import EnumChoiceField
@@ -56,7 +56,7 @@ Migration from 1.x -> 2.x
 
 * Imports of ``EnumFilter`` and ``FilterSet`` has been changed:
 
-    ..code-block: python
+    .. code-block:: python
 
         # 1.x way
         from django_enum import EnumFilter, FilterSet