README: Specify the full path for README.video

It is clearer to specify the full path to access the
doc/README.video file.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
diff --git a/README b/README
index 597d615..88ff837 100644
--- a/README
+++ b/README
@@ -1802,7 +1802,7 @@
 
 		The DIU driver will look for the 'video-mode' environment
 		variable, and if defined, enable the DIU as a console during
-		boot.  See the documentation file README.video for a
+		boot.  See the documentation file doc/README.video for a
 		description of this variable.
 
 - LCD Support:	CONFIG_LCD