ref-manual: Fixed typo for BBMULTICONFIG variable.

There was a typo in the BBMULTICONFIG variable description.
It appeared as "BBMULTIFONFIG". I fixed it.

(From yocto-docs rev: 2ef4ab6d93ccb6208169db9757f9ca2c2551a6d2)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/ref-variables.xml

@@ -1350,7 +1350,7 @@
                     For example, the following line specifies three
                     configuration files:
                     <literallayout class='monospaced'>
-     BBMULTIFONFIG = "configA configB configC"
+     BBMULTICONFIG = "configA configB configC"
                     </literallayout>
                     Each configuration file you use must reside in the
                     <link linkend='build-directory'>Build Directory</link>