cmd: link to doc/usage/fit/x86-fit-boot.rst

Replace the outdated link.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index b026439..6834bbd 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -621,7 +621,7 @@
 
 	  Consider using FIT in preference to this since it supports directly
 	  booting both 32- and 64-bit kernels, as well as secure boot.
-	  Documentation is available in doc/uImage.FIT/x86-fit-boot.txt
+	  Documentation is available in doc/usage/fit/x86-fit-boot.rst.
 
 endmenu