dt: writing-schema: Miscellaneous grammar fixes

  - Add missing verb,
  - Fix accidental plural.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200819124850.20543-1-geert+renesas@glider.be