ref-manual: Fixed icecc example code

Fixes [YOCTO #6912]

The example used to make sure builders use the same sstate
signatures regardless if they use icecc or not was incorrect.
I updated the INHERIT_DISTRO line of the example to use the
append part in the name so it appends the icecc as suggested
by the bug submitter.

Reported-by: Peter Bergin <petan679@gmail.com>
(From yocto-docs rev: 1f6b2f152202ab6e0be3026af3275f67bdb40d16)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/ref-classes.xml

@@ -1177,7 +1177,7 @@
         <link linkend='var-ICECC_DISABLED'><filename>ICECC_DISABLED</filename></link>
         variable to "1" as follows:
         <literallayout class='monospaced'>
-     INHERIT_DISTRO += "icecc"
+     INHERIT_DISTRO_append = " icecc"
      ICECC_DISABLED ??= "1"
         </literallayout>
         This practice makes sure everyone is using the same signatures but also