regulator: pwm-regulator: fix example syntax

The "Continuous Voltage" example specifies a pwm-dutycycle-range.
However, an equal sign is missing between the property name and value.
Fix this to allow copy and paste from the documentation when writing an
own .dts file with a pwm-regulator.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>