dim: Update getting started

We expect a clone now since

commit 2e9ddb93
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Fri Jan 11 14:36:48 2019 +0200

    dim: don't setup maintainer-tools as aux checkout

But somehow the getting started pages haven't been updated.

Acked-by: Jani Nikula <jani.nikula@intel.com>
Cc: Ayan Halder <Ayan.Halder@arm.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>