Update seminar.md

seminar.md

@@ -60,10 +60,10 @@ and a mattermost channel for chat. To join the mattermost channel:
 |2021-03-15| Peter Koepke | Isabelle/Naproche |
 |2021-03-29| André Greiner-Petter | Comparative Verification of Digital Mathematical Libraries and Computer Algebra Systems ([slides](https://gist.github.com/AndreG-P/4fdbd42f313d112c888c10c2136a3620)) |
 |2021-04-12| Deyan Ginev | Scientific Statement Classification over arXiv.org ([slides](https://prodg.org/talks/scientific_statement_classification#1)) |
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 |2021-05-10| ? | ? |
-<!-- |2021-05-24| ? | May be cancelled (public holiday after pentecoast at least in Germany) | -->
 |2021-06-07| ? | ? |
 |2021-07-05| ? | ? |
 
+<!-- |2021-04-26| ? | ? | -->
+<!-- |2021-05-24| ? | May be cancelled (public holiday after pentecoast at least in Germany) | -->