ims_qos_npn/doc/Makefile: proper xml file name

src/modules/ims_qos_npn/doc/Makefile

@@ -1,4 +1,4 @@
-docs = ims_qos.xml
+docs = ims_qos_npn.xml
 
 docbook_dir = ../../../../doc/docbook
 include $(docbook_dir)/Makefile.module