Fixed "nulls characters" typo in docs.

docs/ref/validators.txt

@@ -318,7 +318,7 @@ to, or in lieu of custom ``field.clean()`` methods.
 .. class:: ProhibitNullCharactersValidator(message=None, code=None)
 
     Raises a :exc:`~django.core.exceptions.ValidationError` if ``str(value)``
-    contains one or more nulls characters (``'\x00'``).
+    contains one or more null characters (``'\x00'``).
 
     :param message: If not ``None``, overrides :attr:`.message`.
     :param code: If not ``None``, overrides :attr:`code`.

docs/releases/2.2.4.txt

@@ -70,7 +70,7 @@ Bugfixes
   hand side of an expression is the same type (:ticket:`30621`).
 
 * Fixed a regression in Django 2.2 where auto-reloader crashes if a file path
-  contains nulls characters (``'\x00'``) (:ticket:`30506`).
+  contains null characters (``'\x00'``) (:ticket:`30506`).
 
 * Fixed a regression in Django 2.2 where auto-reloader crashes if a translation
   directory cannot be resolved (:ticket:`30647`).