Documentation: of: Fix grammar usage in graph bindings

Some minor tweaks to the language and formatting used in the
documentation identified while I was reading up on the subject matter.

Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Rob Herring <robh@kernel.org>