ref-manual/variables.rst: fix the description of KBUILD_DEFCONFIG

Remove some leftover confusing mention of KMACHINE.

(From yocto-docs rev: fd6f21b40b7fb58e7b465bbe97b8308f7a09313d)

Signed-off-by: Adam Blank <adam.blank.g@gmail.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
(cherry picked from commit 582ee9e0b4930aea03255542b3d3913df19af8b0)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/ref-manual/variables.rst

@@ -4776,8 +4776,7 @@ system and gives an overview of their function and contents.
       would place patch files and configuration fragment files (i.e.
       "out-of-tree"). However, if you want to use a ``defconfig`` file that
       is part of the kernel tree (i.e. "in-tree"), you can use the
-      :term:`KBUILD_DEFCONFIG` variable and append the
-      :term:`KMACHINE` variable to point to the
+      :term:`KBUILD_DEFCONFIG` variable to point to the
       ``defconfig`` file.
 
       To use the variable, set it in the append file for your kernel recipe