meson: Fix gtk-doc module name

gtk-docs get installed into API versioned subdir (for being able to parallel-install with past and potential future API versions), but unversioned package_name is passed to gnome.gtkdoc. This results in /usr/share/gtk-doc/html/champlain-0.12/champlain.devhelp2 being installed, which the devhelp tool ignores. The devhelp file needs to be named the same as the subdir for it to be picked up, thus use package_string instead of package_name as gtkdoc positional argument to fix it.