javaunohelper: fix javadoc error: unknown tag: returns

Change-Id: I2cd47bd928da322be6c392d65d2a04152553fbef
Reviewed-on: https://gerrit.libreoffice.org/11246
Reviewed-by: Thomas Arnhold <thomas@arnhold.org>
Tested-by: Thomas Arnhold <thomas@arnhold.org>