Add missing word in 'Field.allow_null' docs

Author: Ryan P Kilby

docs/api-guide/fields.md

@@ -45,7 +45,7 @@ Defaults to `True`.
 
 Normally an error will be raised if `None` is passed to a serializer field. Set this keyword argument to `True` if `None` should be considered a valid value.
 
-Note that setting this argument to `True` will imply a default value of `null` for serialization output, but does imply a default for input deserialization.
+Note that setting this argument to `True` will imply a default value of `null` for serialization output, but does not imply a default for input deserialization.
 
 Defaults to `False`