reportbuilder: fix javadoc error: self-closing element not allowed

Change-Id: Iabfa4dd2c1018b982b50798bf09347e0b11901a4
Reviewed-on: https://gerrit.libreoffice.org/11240
Reviewed-by: Thomas Arnhold <thomas@arnhold.org>
Tested-by: Thomas Arnhold <thomas@arnhold.org>