doc: correct references to overlay-fdt-boot.txt

doc/uImage.FIT/overlay-fdt-boot.txt does not exist anymore.
Reference the correct section of doc/usage/fit/overlay-fdt-boot.rst.

Fixes: 6f6e8bb695a7 ("doc: Bring in the FIT overlay information")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/usage/fdt_overlays.rst b/doc/usage/fdt_overlays.rst
index 81d0d37..15ebd17 100644
--- a/doc/usage/fdt_overlays.rst
+++ b/doc/usage/fdt_overlays.rst
@@ -76,8 +76,8 @@
 * Manually load and apply overlays
 
 The remainder of this document will discuss using overlays via the manual
-approach. For information on using overlays as part of a FIT image please see:
-doc/uImage.FIT/overlay-fdt-boot.txt
+approach. For information on using overlays as part of a FIT image please see
+:ref:`fit_configuration_using_overlays`.
 
 Manually Loading and Applying Overlays
 --------------------------------------
diff --git a/doc/usage/fit/overlay-fdt-boot.rst b/doc/usage/fit/overlay-fdt-boot.rst
index a7db1a3..3d7296a 100644
--- a/doc/usage/fit/overlay-fdt-boot.rst
+++ b/doc/usage/fit/overlay-fdt-boot.rst
@@ -111,6 +111,8 @@
 
 This selects the DTB to use when booting.
 
+.. _fit_configuration_using_overlays:
+
 Configuration using overlays
 ----------------------------