use theano.nnet.conv2d instead of theano.nnet.conv.conv2d

[diogo149]

Had a friend play with the tutorial and ran into the issue described here: Theano/Theano#3992

It might be friendlier for new people who want to play around with the code use theano.nnet.conv2d. If there's interest , I'd happily make a PR (though feel free to make the one line change without me) if you feel like it's the right thing to do.