Update extending_theano.txt

a9fc4620 · Frédéric Bastien · GitHub · 89d592e3 · a9fc4620
--- a/doc/extending/extending_theano.txt
+++ b/doc/extending/extending_theano.txt
@@ -791,7 +791,7 @@ implementation.
 It takes an optional :func:`infer_shape` parameter that must have this
 signature:

-.. code-block::
+.. code-block:: none

    def infer_shape(node, input_shapes):
        # ...