Edits to convolutional multilayer perceptron tutorial, for clarity and grammar.

That's not really accurate. `ConvOp` is used both by `theano.tensor.nnet.conv2d`, which deals with 4D tensors and is used for CNNs, and by `theano.tensor.signal.conv2d` which replicates the behaviour of `scipy.signal.convolve2d`.
I think we can just drop the reference to the scipy function.