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Fix undo shortcut in Undoing.md (#5640)

We fixed this recently in the Readme (see 7a3bae4de1f7), but forgot to
update this.

Fixes #5630.
S
Stefan Haller committed
21b59f9e333030608ea71c496578de2167955309
Committed by GitHub <noreply@github.com> on 5/26/2026, 7:49:16 AM