regulator: max8997: Fix a trivial typo in documentation

Changed volatage to voltage.
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>

Documentation/devicetree/bindings/regulator/max8997-regulator.txt

 * Maxim MAX8997 Voltage and Current Regulator
 
-The Maxim MAX8997 is a multi-function device which includes volatage and
+The Maxim MAX8997 is a multi-function device which includes voltage and
 current regulators, rtc, charger controller and other sub-blocks. It is
 interfaced to the host controller using a i2c interface. Each sub-block is
 addressed by the host system using different i2c slave address. This document