bitbake: user-manual-metadata.xml: Renamed section tag

There were two section tags that used the same id name "classes".
One was in the intro chapter and one was in the metadata chapter.
This was causing an exception in the PDF creation process and
breaking it.  I renamed the tag in the metadata chapter
"metadata-classes".

(Bitbake rev: d5f405586f7c50c602241519d32d02b1c7b1f345)

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

bitbake/doc/user-manual/user-manual-metadata.xml

@@ -779,7 +779,7 @@
             </section>
         </section>
 
-        <section id='classes'>
+        <section id='metadata-classes'>
             <title>Classes</title>
 
             <para>