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/README.pxe b/doc/README.pxe
index af2e64a..9fff2cd 100644
--- a/doc/README.pxe
+++ b/doc/README.pxe
@@ -160,7 +160,7 @@
 		      It will passed to bootm with that address.
 		      (see: doc/uImage.FIT/command_syntax_extensions.txt)
 		      It useful for overlay selection in pxe file
-		      (see: doc/uImage.FIT/overlay-fdt-boot.txt)
+		      (see: doc/usage/fit/overlay-fdt-boot.rst).
 
 fdtoverlays <path> [...] - if this label is chosen, use tftp to retrieve the DT
                       overlay(s) at <path>. it will be temporarily stored at the