doc: README: Correct file name of signature verification documentation

Signed-off-by: Detlev Zundel <dzu@denx.de>
diff --git a/README b/README
index 44ce071..bfd89b0 100644
--- a/README
+++ b/README
@@ -2850,7 +2850,7 @@
 		CONFIG_RSA
 
 		This enables the RSA algorithm used for FIT image verification
-		in U-Boot. See doc/uImage/signature for more information.
+		in U-Boot. See doc/uImage.FIT/signature.txt for more information.
 
 		The signing part is build into mkimage regardless of this
 		option.