fix userguide doctests

[bjlittle]

I re-ran the doc tests to recreate the issue, then fixed the problem. Just so that you know what I done, incase you need to do some thing similar...

I created a conda environment from the conda list --explicit output in the travis log.

Activated the environment, branched from your branch, then...

> cd docs/iris
> make clean
> make doctest

then fixed the tests

HTH 😄

[ocefpaf]

Thanks @bjlittle!