fix up docs in Getting started

[Francis J. Poulin]

I am looking at the docs and see that Function and Array also needs to be imported for the example to execute.

from shenfun import Basis, Function, Array
N = 8
T = Basis(N, 'Chebyshev', bc=None)

Also, later on in the code right before Operators you need to import Dx for this code to work.

From the Multipdimensional part, one needs to import TensorProductSpace, define comm and also import grad.

In coupled problems, the line beginning with SD does not work since int is not scriptable.

This is as far as I have gotten now. Will try and look at the rest another time.