docs.bzl: Use absolute path for cp
docs.bzl: Use absolute path for cp

This works externally with relative paths but fails in our internal
CI system. We need the absolute path of the tool, even if it is cp.

Change-Id: Ie9af9efc3c456dd3bb29a00d137e1fa375a9659b

google/copybara,google/copybara,google/copybara