d/rules: Fix spelling of desktop_docs Meson option
This was accepted with a warning under Meson 0.59, but causes FTBFS under Meson 0.60.
Closes: #998584
Backport fix from, https://salsa.debian.org/gnome-team/gnome-desktop/-/commit/f2448f68be775196bba5ec83f1955a88906cf5f9