regulator: palmas-pmic: doc: fix typo for sleep-mode

commit 3c870e3f
(regulator: palmas: Change the DT node property names to follow the convention)

Missed updating mode-sleep from sleep-mode. Fix the same.

Documentation example seems proper for this property.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>