fix 3d demo doc

Author: Ben-Louis

demo/docs/en/3d_human_pose_demo.md

@@ -64,17 +64,16 @@ https://download.openmmlab.com/mmpose/body3d/videopose/videopose_h36m_243frames_
 During 2D pose detection, for multi-frame inference that rely on extra frames to get the final results of the current frame, try this:
 
 ```shell
-python  demo/body3d_pose_lifter_demo.py  \
-python demo/body3d_pose_lifter_demo.py  \
+python demo/body3d_pose_lifter_demo.py \
 demo/mmdetection_cfg/rtmdet_m_640-8xb32_coco-person.py \
 https://download.openmmlab.com/mmpose/v1/projects/rtmpose/rtmdet_m_8xb32-100e_coco-obj365-person-235e8209.pth \
 configs/body_2d_keypoint/rtmpose/body8/rtmpose-m_8xb256-420e_body8-256x192.py \
 https://download.openmmlab.com/mmpose/v1/projects/rtmposev1/rtmpose-m_simcc-body7_pt-body7_420e-256x192-e48f03d0_20230504.pth \
 configs/body_3d_keypoint/pose_lift/h36m/pose-lift_videopose3d-243frm-supv-cpn-ft_8xb128-200e_h36m.py \
-https://download.openmmlab.com/mmpose/body3d/videopose/videopose_h36m_243frames_fullconv_supervised_cpn_ft-88f5abbb_20210527.pth  \
+https://download.openmmlab.com/mmpose/body3d/videopose/videopose_h36m_243frames_fullconv_supervised_cpn_ft-88f5abbb_20210527.pth \
 --input https://user-images.githubusercontent.com/87690686/164970135-b14e424c-765a-4180-9bc8-fa8d6abc5510.mp4 \
---output-root  vis_results  \
---use-multi-frames  --online
+--output-root vis_results \
+--use-multi-frames --online
 ```
 
 ### 3D Human Pose Demo with Inferencer