DOC: Improve example on array broadcasting

[arunppsg]

The earlier version of doc, it read indexing an array of shape (10, 20, 30)
with an index shaped (2, 3, 4) but it will throw an IndexError because
the (20,)-shaped subspace cannot be replaced with (2, 3, 4) shaped
broadcasted index subspace.

I updated the example by replacing (2, 3, 4) with (2, 5, 2) to make
the index compatible with the subspace.

[mattip]

Thanks @arunppsg