dt-bindings: example: Grammar improvements

Improve grammar in recently added sentences:
  - Drop superfluous "be" in front of "have",
  - Add missing articles.

Fixes: 73f76a41 ("dt-bindings: example: Extend based on practice")
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200925111429.3146-1-geert+renesas@glider.be


Signed-off-by: Rob Herring <robh@kernel.org>