Remove stray period in slides

[welcome[bot]]

Thanks for opening your first pull request here! If you have any questions, feel free to mention one of the conveners, previous contributors, or attend our weekly meeting (see https://hepsoftwarefoundation.org/workinggroups/training.html). Also, sometimes PRs go unnoticed, so don't hesitate to @mention some of us, if we do not come back to you within a few days.

[welcome[bot]]

Congrats on merging your first pull request! We at HSF are proud of you!

[hageboeck]

Thanks!