docs: escape cp for PROJECT_DIR with spaces

[skimi]

Small update to the doc to take into account that PROJECT_DIR may contain spaces and should be "escaped" so that the cp works in those cases.

I had the issue on my project where it worked well locally and failed in the CI because the CI had a space in PROJECT_DIR.

Since like me most people will copy paste the cp line from the readme it's worthwhile updating it so less people get this issue

[height]

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[luancurti]

Thanks!