contrib: Fix help message in build_imgs_package.sh
contrib: Fix help message in build_imgs_package.sh

The fourth argument (build dir) was incorrectly documented.

Signed-off-by: Martin Braun <3d56405a74868c0c3ff5cba6e886d82741a922ab@ettus.com>

EttusResearch/meta-ettus,EttusResearch/meta-ettus,EttusResearch/meta-ettus,EttusResearch/meta-ettus,EttusResearch/meta-ettus,EttusResearch/meta-ettus