Fix include path error in mkdocs.sh introduced in 247cd3381745

Unfortunately in commit 247cd33817457789645827a52d79a1876b9760a3 I
mistakenly included the directories wrongly. This fixes this.

Change-Id: I91193321d1264985eab4865ca05adeab96401b2f
Reviewed-on: https://gerrit.libreoffice.org/7669
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>