Fix doc again.

doc/tutorial/extending_theano_c.txt

@@ -997,7 +997,7 @@ down computation on the GPU, but it is enabled by default on the CPU).
 Then you must start Python inside GDB and in it start your Python
 process (e.g. theano-nose):
 
-.. code-block::
+.. code-block:: sh
 
     $gdb python
     (gdb)r bin/theano-nose theano/