Documentation: earlycon: fix Marvell Armada 3700 UART name

The Marvell Armada 3700 UART uses "ar3700_uart" for its earlycon name.
Adjust documentation to match the code.
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/kernel-parameters.txt

@@ -972,7 +972,7 @@
 			A valid base address must be provided, and the serial
 			port must already be setup and configured.
 
-		armada3700_uart,<addr>
+		ar3700_uart,<addr>
 			Start an early, polled-mode console on the
 			Armada 3700 serial port at the specified
 			address. The serial port must already be setup