HTML docs are installed to both $docdir and $pkgdatadir. Fix that
to install to $docdir only.
Change-Id: I115158585b6df9170d9a01249adbc8548df91f14
Reviewed-on: https://code.wireshark.org/review/34640
Petri-Dish: João Valverde <j@v6e.pt>
Tested-by: Petri Dish Buildbot
Reviewed-by: João Valverde <j@v6e.pt>