Fix Windows HTML visualization to make videos could be seen

[ctbfl]

What this does

Fix a 🐛 Bug. I found that on Windows the HTML dataset visualize does not work well, as the the videos can NOT be seen.

Then I analyse and find it's the reason of file path format.

After fix two lines, I get the thing right.

How to checkout & try? (for the reviewer)

Examples: (on windows, tested by myself)

python .\lerobot\scripts\visualize_dataset_html.py --repo-id lerobot/pusht

(on linux/mac, not tested, but must be fine)

python ./lerobot/scripts/visualize_dataset_html.py --repo-id lerobot/pusht

[ctbfl]

@Cadene @aliberts

[aliberts]

@ctbfl do you still have this issue?

@mishig25 ^ wdyt?