Made load_ckpt_path relative in demo/demo.yaml

[Arqam-Siddiqi]

Problem: The demo/demo.yaml file contained an absolute path to the checkpoint file on the author's PC, like so: "/Workspace/ZhaoYuzhong/ControlCap/output/train/vg_refcocog_5e/20240316102/checkpoint_4.pth"

Fix: The demo/demo.yaml file now contains a relative path to the checkpoint file, which will work for all users.

This is also more in line with the documentation in docs/data.md.