Update URL reference material in /contrib/isn docs

Report by Peter Eisentraut

doc/src/sgml/isn.sgml

@@ -364,7 +364,9 @@ SELECT isbn13(id) FROM test;
    The prefixes used for hyphenation were also compiled from:
    <itemizedlist>
     <listitem><para><ulink url="http://www.gs1.org/productssolutions/idkeys/support/prefix_list.html"></ulink></para></listitem>
-    <listitem><para><ulink url="http://www.isbn-international.org/en/identifiers.html"></ulink></para></listitem>
+    <listitem><para><ulink url="http://en.wikipedia.org/wiki/List_of_ISBN_identifier_groups"></ulink></para></listitem>
+    <listitem><para><ulink url="https://www.isbn-international.org/content/isbn-users-manual"></ulink></para></listitem>
+    <listitem><para><ulink url="http://en.wikipedia.org/wiki/International_Standard_Music_Number"></ulink></para></listitem>
     <listitem><para><ulink url="http://www.ismn-international.org/ranges.html"></ulink></para></listitem>
    </itemizedlist>