Indentation errors in the .readthedocs.yml example file

[alexbinks]

To the authors: this is a great guide! I've been learning how
to get sphinx to produce documentation and it's working great!

The section entitled Setting up ReadTheDocs contains some
code for the .readthedocs.yml file to be used when setting
up the ReadTheDocs documentation.

However, it seems there is a problem with the indentation in
the example provided, and when I tried to build the file using
this I ran into a set of compilation errors.

I cloned the repo and made a fix to the in the docs/docs.rst file
and this now compiles OK.

The link is here.

Hopefully this can help people using the guide!

Cheers,

Alex