nitrogen6x: README: Do not refer to MAINTAINERS file

MAINTAINERS file has been removed from the project.

Replace its reference with 'boards.cfg' file instead.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Eric Nelson <eric.nelson@boundarydevices.com>
diff --git a/board/boundary/nitrogen6x/README b/board/boundary/nitrogen6x/README
index 6f449be..9d84265 100644
--- a/board/boundary/nitrogen6x/README
+++ b/board/boundary/nitrogen6x/README
@@ -70,7 +70,7 @@
 	make
 
 Note that 'nitrogen6x' is a placeholder. The complete list of supported
-board configurations is shown in tha MAINTAINERS file:
+board configurations is shown in the boards.cfg file:
 	nitrogen6q		i.MX6Q/6D	1GB
 	nitrogen6dl		i.MX6DL		1GB
 	nitrogen6s		i.MX6S		512MB