Fix copy-n-paste example to be clearer.

The example for spinning up an etherpad server couldn't just be run,
because the path to install_puppet.sh and the path to local.pp were
inconsistent. So lets document that the commands should be run from
the root of the checkout and fix the path to local.pp.

Change-Id: I34d4768687bd8aea8cd695faa845bddef24fa55f
Merge "Fix copy-n-paste example to be clearer."