document/poky-ref-manual/ref-classes.xml: removed <function> tag

I got rid of the <function> tag and replaced with <filename>.  We
have too many styles.

(From OE-Core rev: 5ac97ba191c8707ff20105626427998df997d221)

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

documentation/poky-ref-manual/ref-classes.xml

@@ -35,7 +35,7 @@
         (empty by default). These are often overridden or extended by other classes 
         such as <filename>autotools.bbclass</filename> or 
         <filename>package.bbclass</filename>. The class also contains some commonly
-        used functions such as <function>oe_runmake</function>.
+        used functions such as <variable>oe_runmake</variable>.
     </para>
 </section>