dt-bindings: example-schema: Standard unit should be microvolt not microvolts

Even the DT maintainer gets confused. The schema in dt-schema was wrong
too, so this was passing validation until trying to add some common
incorrect patterns to check.

Fixes: 58fbe999 ("dt-bindings: example-schema: Add some additional examples and commentary")
Signed-off-by: Rob Herring <robh@kernel.org>