doc: fix GSO guide

doc: fix GSO guide

Correct two minor issues in the GSO programmer's guide:
- a note is rendered incorrectly in the middle of an unordered list;
  this results in the remainder of the list appearing inside the note.
  Correct indentation of the note to resolve same.
- two minor visual artifacts are present in the 'three-part-output-segment'
  diagram. Remove same.

Fixes: f6010c7655cc ("doc: add GSO programmer's guide")

Signed-off-by: Mark Kavanagh <mark.b.kavanagh@intel.com>
Acked-by: John McNamara <john.mcnamara@intel.com>