fix id

Author: bnmnetp

pretext/backfaq.ptx

@@ -7,7 +7,7 @@
 <!-- Copyright (C) 2013-2018  Robert A. Beezer                -->
 <!-- See the file COPYING for copying conditions.             -->
 
-<appendix xml:id="welcome"
+<appendix xml:id="faq_appendix"
     xmlns:xi="http://www.w3.org/2001/XInclude">
     <title>Frequently Asked Questions</title>
 
@@ -18,4 +18,4 @@
     <xi:include href="./faq.ptx" />
 
 
-</appendix>
+</appendix>