doc: Update link

The reference "That is the topic of the next section." has been
incorrect since the materialized views documentation got inserted
between the section "rules-views" and "rules-update".

Author: Zertrin <postgres_wiki@zertrin.org>

doc/src/sgml/rules.sgml

@@ -862,7 +862,7 @@ SELECT t1.a, t2.b, t1.ctid FROM t1, t2 WHERE t1.a = t2.a;
     <command>UPDATE</command>, and <command>DELETE</command> commands on
     a view. These rules will rewrite the command, typically into a command
     that updates one or more tables, rather than views. That is the topic
-    of the next section.
+    of <xref linkend="rules-update">.
 </para>
 
 <para>