docs(troubleshooting): Update docs about copying file for setuid sandbox

[WillGibson]

Very small change in light of operational experience while getting it running on Centos in Jenkins pipeline.

Without the -p, the permissions set in the chmod before this command are not carried over chrome cannot start.

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[WillGibson]

I signed it!

[googlebot]

CLAs look good, thanks!

[JoelEinbinder]

Thanks for the PR! Can you add to the description about what this does/why it is needed?

[WillGibson]

Of course, I was rushing yesterday to get out of work in time for yoga. Done.