Documentation: Use correct file name for the build guide in the Makefile

Change-Id: I19c456e8bcd2de19c5f9d963ea17dad84d300ab8
Signed-off-by: Vladimir Berezniker <vmpn@vmpn.net>
Reviewed-on: http://review.coreboot.org/6170
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <gaumless@gmail.com>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>