dt: Fix incorrect reference in gpio-led documentation

Path to gpio.txt (dt) document was broken.
Signed-off-by: NSachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/gpio/led.txt

@@ -8,7 +8,7 @@ node's name represents the name of the corresponding LED.
 
 LED sub-node properties:
 - gpios :  Should specify the LED's GPIO, see "gpios property" in
-  Documentation/devicetree/gpio.txt.  Active low LEDs should be
+  Documentation/devicetree/bindings/gpio/gpio.txt.  Active low LEDs should be
   indicated using flags in the GPIO specifier.
 - label :  (optional) The label for this LED.  If omitted, the label is
   taken from the node name (excluding the unit address).