mips: update a reference to a moved Arm Document

Arm documentation has moved to Documentation/arch/arm; update a reference
in arch/mips/bmips/setup.c to match.

Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: linux-mips@vger.kernel.org
Acked-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

arch/mips/bmips/setup.c

@@ -178,7 +178,10 @@ void __init plat_mem_setup(void)
 	ioport_resource.start = 0;
 	ioport_resource.end = ~0;
 
-	/* intended to somewhat resemble ARM; see Documentation/arm/booting.rst */
+	/*
+	 * intended to somewhat resemble ARM; see
+	 * Documentation/arch/arm/booting.rst
+	 */
 	if (fw_arg0 == 0 && fw_arg1 == 0xffffffff)
 		dtb = phys_to_virt(fw_arg2);
 	else