information on DATA.md is wrong. Update info on data preparation

[Q-Tran]

python3 launch.py --np 1 --nn 0 --eval-only --uni 1 --config-file projects/UNINEXT/configs/eval-vid/video_joint_r50_eval_ovis.yaml --resume OUTPUT_DIR outputs/${EXP_NAME} MODEL.USE_IOU_BRANCH False

When running the following the console tell me that the filestructure for ovis should be dataset/ovis/valid instead of dataset/ovis/val. Also important is that the ovis validation data json given lists annotations as null and a annotations nonetype error comes up. Upon further inspection the youtube vis data doesn't have this in their json. simply deleting the annotations key value in the ovis json fixes the problem.