BUG/SMALL - Fix github link instructions on exp-lookit-video

[mekline]

The current instructions are confusing because they use MIT-hosted stimuli, which don't model the issue with Github URLs. Instead, use the sample_video files already contained in the GH stimuli repository!

(In short links should be formatted as:

https://raw.githubusercontent.com/user/repo/branch

not

https://github.com/user/repo/ )

[mekline]

NOTE: Better instructions exist in the template repository readme, but this is not clear from the docs - repeat or explicitly reference GH URL formatting issue from here: https://github.com/lookit/lookit-stimuli-template/blob/master/README.md