Update spdx.go help message
Update spdx.go help message

Signed-off-by: Vlad Velici <a8015d957ed6ac771323a12f644aff1a0d97689a@gmail.com>

vladvelici/spdx-go,spdx/tools-go,spdx/tools-go,spdx/tools-go,vladvelici/spdx-go