fix docs build fix make distcheck

Original commit message from CVS:
fix docs build
fix make distcheck
diff --git a/docs/pwg/pwg.xml b/docs/pwg/pwg.xml
index c60f1d8..be67573 100644
--- a/docs/pwg/pwg.xml
+++ b/docs/pwg/pwg.xml
@@ -38,7 +38,7 @@
 <!ENTITY GstLibRef    "<emphasis>GStreamer Library Reference</emphasis>">
 ]>
 
-<book id="book-gst-plugin-writers-guide">
+<book id="index">
   &TITLEPAGE;
 
   <!-- ############# part ############### -->