regulator: bindings: Use the correct symbol for second

The symbol for second is in lower case.
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/regulator.txt

@@ -13,7 +13,7 @@ Optional properties:
 - regulator-allow-bypass: allow the regulator to go into bypass mode
 - regulator-allow-set-load: allow the regulator performance level to be configured
 - <name>-supply: phandle to the parent supply/regulator node
-- regulator-ramp-delay: ramp delay for regulator(in uV/uS)
+- regulator-ramp-delay: ramp delay for regulator(in uV/us)
   For hardware which supports disabling ramp rate, it should be explicitly
   initialised to zero (regulator-ramp-delay = <0>) for disabling ramp delay.
 - regulator-enable-ramp-delay: The time taken, in microseconds, for the supply