Add missing commas to the contrib.sla module code examples.

[aliavni]

Description

Code examples in the luigi.contrib.sqla module were misleading with the missing commas in tuples.

Motivation and Context

This code example is copied and pasted in other resources than the Luigi documentation (e.g., http://www.tuicool.com/articles/bumiYr). The motivation here is a better documentation.

Have you tested this? If so, how?

I built the documentation with tox -e docs and confirmed the changes.

[mention-bot]

@aliavni, thanks for your PR! By analyzing the history of the files in this pull request, we identified @Tarrasch, @soxofaan and @graingert to be potential reviewers.

[graingert]

Any way of testing this?

[aliavni]

@graingert do you mean other than building the docs with tox -e docs and confirming the changes?

[graingert]

doctest? Run a linter over the code blocks?

[Tarrasch]

@graingert , do you want to take a short look at https://pypi.python.org/pypi/flake8-docstrings? It seems all you need to do is to add a line to the tox file. I saw this example but I think it's outdated.