Fix compliance with Atom specification

[ilpianista]

With commit db0164f the code that generates the atom:updated element has been changed from time::now_utc().rfc3339().to_string() to chrono::Utc::now().to_rfc3339(), but unfortunately the latter isn't a replacement of the first because it includes the nanoseconds in its output:

2009-02-13T15:31:30-08:00 (time)
2020-05-12T14:24:08.866226229+00:00 (chrono)

The Atom specification defines the updated element of type xsd:dateTime which doesn't include the nanoseconds.

I discovered this by using feed-io to parse your Atom feed.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @nikomatsakis (or someone else) soon.

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[ilpianista]

Maybe there's no need to change this, but I thought it was OK to keep them aligned.

[XAMPPRocky]

Thank you for your PR, and congrats on your first contribution! 🎉