testcode for doc/library/compile/debugmode.txt

doc/library/compile/debugmode.txt

@@ -25,8 +25,12 @@ a cluster.
 
 DebugMode can be used as follows:
 
-.. code-block:: python
+.. testcode::
 
+    import theano
+    from theano import tensor
+    from theano.compile.debugmode import DebugMode
+    
     x = tensor.dscalar('x')
 
     f = theano.function([x], 10*x, mode='DebugMode')