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>

README

@@ -1802,7 +1802,7 @@ CBFS (Coreboot Filesystem) support
 
 		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