doc: make path platform dependent

Path should now appear with the correct slash, depending on which
platform install document is being viewed - keeping in line with the
rest of the document.

Fixes #30160

Change-Id: Ib10e5a4adf366c700bff6f8d246bd5e3111ed61c
Reviewed-on: https://go-review.googlesource.com/c/go/+/162918


Reviewed-by: Andrew Bonventre <andybons@golang.org>