fix typo

tutos/python_data_types.md

@@ -103,7 +103,7 @@ You can assign values to an instance (i.e. object) using __dot notation__:
 ### Objects are mutable
 
 ```python
-def translate_point(point, dx, dy)
+def translate_point(point, dx, dy):
     point.x += dx
     point.y += dy
 ```