Fix bug in Event example

Author: siddhantwahal

slides/02_traits.md

@@ -515,15 +515,15 @@ type(c.age)
 <!-- #endregion -->
 
 ```python
-from traits.api import HasTraits
+from traits.api import Event, File, HasStrictTraits, Instance, observe, Str
 
 class DataFile(HasStrictTraits):
     file = File
     data_changed = Event
 
 
 class DataReader(HasStrictTraits):
-    file = DataFile
+    file = Instance(DataFile)
     content = Str
 
     @observe("file.data_changed")
@@ -540,7 +540,6 @@ r = DataReader(file=f)
 f.data_changed = True
 ```
 
-
 <!-- #region slideshow={"slide_type": "slide"} -->
 ## Exercise time!