DOC: Add syntax highlighting to SAS code blocks in comparison_with_sas.rst
Since version 2.2, Pygments supports [SAS syntax highlighting](http://pygments.org/docs/lexers/#lexer-for-sas). This PR just changes `.. code-block:: none` to `.. code-block:: sas`.

In my local build, that causes the code chunks to look like this:
![image](https://user-images.githubusercontent.com/15164633/37216851-de794a62-2389-11e8-9a17-66dc2dcd880e.png)


Checklist for other PRs (remove this part if you are doing a PR for the pandas documentation sprint):

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
