ref-manual: variables: fixup class reference in IMAGE_MANIFEST

The definition of IMAGE_MANIFEST was moved to a different class. Fixup
the documentation to reflect this change.

(From yocto-docs rev: 15c0a09154de2a1f6d36e1bb5bf283bd08c9ec3d)

Signed-off-by: Daniel Wagenknecht <dwagenknecht@emlix.com>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/variables.rst

@@ -3099,7 +3099,7 @@ system and gives an overview of their function and contents.
 
           packagename packagearch version
 
-      The :ref:`image <ref-classes-image>` class defines the manifest
+      The :ref:`rootfs-postcommands <ref-classes-rootfs*>` class defines the manifest
       file as follows::
 
          IMAGE_MANIFEST ="${IMGDEPLOYDIR}/${IMAGE_NAME}${IMAGE_NAME_SUFFIX}.manifest"