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>
3 files changed