Improve CalendarBase documentation

Fix copy-paste error
4 jobs for master
Status Job ID Name Coverage
  Build
passed #92553
build_clazy_clang_tidy

00:06:14

passed #92552
linux kf5-qt5 qt5.15

00:04:40

1.0%
failed #92485
build_clazy_clang_tidy

failed #92484
linux kf5-qt5 qt5.15