boot: Fix reference to bootmenu doc

The Kconfig references a readme file that's moved and
converted to rst so update the reference.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/Kconfig b/boot/Kconfig
index e8fb03b..86c2787 100644
--- a/boot/Kconfig
+++ b/boot/Kconfig
@@ -1439,7 +1439,7 @@
 	  This enables the boot menu, controlled by environment variables
 	  defined by the board. The menu starts after running the 'preboot'
 	  environmnent variable (if enabled) and before handling the boot delay.
-	  See README.bootmenu for more details.
+	  See doc/usage/cmd/bootmenu.rst for more details.
 
 config BOOTMENU_DISABLE_UBOOT_CONSOLE
 	bool "Disallow bootmenu to enter the U-Boot console"