Doc: Correct deduplicate_items varlistentry id.

Use a varlistentry id for the deduplicate_items storage parameter that
is derived from the name of the parameter itself.

This oversight happened because the storage parameter was renamed
relatively late during the development of the patch that became commit
0d861bb.

Author: petergeoghegan

doc/src/sgml/ref/create_index.sgml

@@ -399,7 +399,7 @@ CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ [ IF NOT EXISTS ] <replaceable class=
    </para>
 
    <variablelist>
-   <varlistentry id="index-reloption-deduplication" xreflabel="deduplicate_items">
+   <varlistentry id="index-reloption-deduplicate-items" xreflabel="deduplicate_items">
     <term><literal>deduplicate_items</literal> (<type>boolean</type>)
      <indexterm>
       <primary><varname>deduplicate_items</varname> storage parameter</primary>