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>

Documentation/devicetree/bindings/example-schema.yaml

@@ -160,7 +160,7 @@ properties:
           - enum: [ foo, bar ]
           - enum: [ baz, boo ]
 
-  vendor,property-in-standard-units-microvolts:
+  vendor,property-in-standard-units-microvolt:
     description: Vendor specific properties having a standard unit suffix
       don't need a type.
     enum: [ 100, 200, 300 ]