docs: fix imitation learning robots docs command

Author: imstevenpmwork

docs/source/il_robots.mdx

@@ -301,7 +301,7 @@ python -m lerobot.record  \
   --robot.cameras="{ up: {type: opencv, index_or_path: /dev/video10, width: 640, height: 480, fps: 30}, side: {type: intelrealsense, serial_number_or_name: 233522074606, width: 640, height: 480, fps: 30}}" \
   --robot.id=my_awesome_follower_arm \
   --display_data=false \
-  --dataset.repo_id=$HF_USER/eval_so100 \
+  --dataset.repo_id=${HF_USER}/eval_so100 \
   --dataset.single_task="Put lego brick into the transparent box" \
   # <- Teleop optional if you want to teleoperate in between episodes \
   # --teleop.type=so100_leader \