Fix typos.

docs/api-guide/relations.md

@@ -302,7 +302,7 @@ Note that reverse relationships are not automatically generated by the `ModelSer
 Instead, you must explicitly add it to the serializer.  For example:
 
     class AlbumSerializer(serializers.ModelSerializer):
-        tracks = serializers.PrimaryKeyRelationship(many=True)
+        tracks = serializers.PrimaryKeyRelatedField(many=True)
         ...
 
 By default, the field will uses the same accessor as it's field name to retrieve the relationship, so in this example, `Album` instances would need to have the `tracks` attribute for this relationship to work.
@@ -316,7 +316,7 @@ The best way to ensure this is typically to make sure that the relationship on t
 Alternatively, you can use the `source` argument on the serializer field, to use a different accessor attribute than the field name.  For example.
 
     class AlbumSerializer(serializers.ModelSerializer):
-        tracks = serializers.PrimaryKeyRelationship(many=True, source='track_set')
+        tracks = serializers.PrimaryKeyRelatedField(many=True, source='track_set')
 
 See the Django documentation on [reverse relationships][reverse-relationships] for more details.