Fix include path error in mkdocs.sh introduced in 247cd338

Unfortunately in commit 247cd338 I
mistakenly included the directories wrongly. This fixes this.

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

solenv/bin/mkdocs.sh

@@ -164,7 +164,7 @@ do
   fi
 
   if [ -d "include/$PROJECT" ]; then
-      PROJECT_INCLUDE="$PROJECT_INCLUDE $PROJECT_INCLUDE"
+      PROJECT_INCLUDE="$PROJECT_INCLUDE include/$PROJECT"
   fi
 
   DOXYGEN_INPUT=`printf "%s" "$PROJECT/source $PROJECT_INCLUDE"`