reportbuilder: fix javadoc error: reference not found

Change-Id: Ic548a23a9d1e1acc90386da76416c76990f3a5b3
Reviewed-on: https://gerrit.libreoffice.org/11309
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>