testcode for doc/tutorial/symbolic_graphs.txt

doc/tutorial/symbolic_graphs.txt

@@ -34,8 +34,9 @@ detail about these building blocks refer to :ref:`variable`, :ref:`op`,
 
 **Code**
 
-.. code-block:: python
+.. testcode::
 
+    import theano.tensor as T
     x = T.dmatrix('x')
     y = T.dmatrix('y')
     z = x + y