bpo-30312: Small correction in datastructures set code sample

[KatherineMichel]

No description provided.

[mention-bot]

@KatherineMichel, thanks for your PR! By analyzing the history of the files in this pull request, we identified @birkenfeld, @ezio-melotti and @tiran to be potential reviewers.

[the-knights-who-say-ni]

Hello, and thanks for your contribution!

I'm a bot set up to make sure that the project can legally accept your contribution by verifying you have signed the PSF contributor agreement (CLA).

Unfortunately we couldn't find an account corresponding to your GitHub username on bugs.python.org (b.p.o) to verify you have signed the CLA (this might be simply due to a missing "GitHub Name" entry in your b.p.o account settings). This is necessary for legal reasons before we can look at your contribution. Please follow the steps outlined in the CPython devguide to rectify this issue.

Thanks again to your contribution and we look forward to looking at it!

[KatherineMichel]

My CLA is now signed.

[Mariatta]

Thanks @KatherineMichel . Can you add your GitHub username to your account in bugs.python.org?

example: https://cloud.githubusercontent.com/assets/2680980/23276970/d14a380c-f9d1-11e6-883d-e13b6b211239.png

[Mariatta]

Thanks @KatherineMichel and congrats on your first contribution to CPython 🎉