Fix X-DocPath entries

This way they're more correct, which means they can work with KHelpCenter
and when falling back to opening the relevant page at docs.kde.org in any
web browser.

See graphics/okular!441 for more
details.
2 jobs for master in 9 minutes and 48 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Build
passed build_clazy_clang_tidy #111643

00:06:37

passed linux kf5-qt5 qt5.15 #111642

00:03:11

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