BUGFIX: tutorial -> Derivatives in Theano -> Jacobian times a Vector -> L-operator.

I've fixed a bug in tutorial described here https://groups.google.com/forum/#!topic/theano-users/nC2l8e-WL3c/discussion
Error text: "UnusedInputError: theano.function was asked to create a function computing outputs given certain inputs, but the provided input variable at index 0 is not part of the computational graph needed to compute the outputs: W. To make this error into a warning, you can pass the parameter on_unused_input='warn' to theano.function. To disable it completely, use on_unused_input='ignore'."