ref-manual: kernel-fitimage.bbclass does not use SPL_SIGN_KEYNAME

The kernel-fitimage.bbclass uses the UBOOT_* variables. The SPL_*
variables are handled by uboot-sign.bbclass.

(From yocto-docs rev: 8ceffee908a039deb1021361faa7637e83ef26f3)

Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/variables.rst

@@ -8766,7 +8766,7 @@ system and gives an overview of their function and contents.
       class.
 
    :term:`SPL_SIGN_KEYNAME`
-      The name of keys used by the :ref:`ref-classes-kernel-fitimage` class
+      The name of keys used by the :ref:`ref-classes-uboot-sign` class
       for signing U-Boot FIT image stored in the :term:`SPL_SIGN_KEYDIR`
       directory. If we have for example a ``dev.key`` key and a ``dev.crt``
       certificate stored in the :term:`SPL_SIGN_KEYDIR` directory, you will