dim: Fix assorted documentation typos

Just a bunch of trivial typos that caught my eye while perusing the
documentation.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>