reportbuilder: fix javadoc error: unknown tag: returns

Change-Id: I3cc2afca6e59a8bc9e3374e5cebf9b2e903da78b
Reviewed-on: https://gerrit.libreoffice.org/11241
Reviewed-by: Thomas Arnhold <thomas@arnhold.org>
Tested-by: Thomas Arnhold <thomas@arnhold.org>