pinctrl: dt-bindings: Fix A37xx uart2 group name

Fix a typo in A37xx pin controllers documentation about uart2 pin group.
Signed-off-by: NMiquel Raynal <miquel.raynal@free-electrons.com>
Reviewed-by: NGregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/pinctrl/marvell,armada-37xx-pinctrl.txt

@@ -97,8 +97,8 @@ group spi_quad
  - pins 15-16
  - functions spi, gpio
 
-group uart_2
- - pins 9-10
+group uart2
+ - pins 9-10 and 18-19
  - functions uart, gpio
 
 Available groups and functions for the South bridge: