Doc: Fix deduplicate_items index term.

Reported-By: Fujii Masao
Discussion: https://postgr.es/m/18f07ae8-7d89-537c-b0a9-54100a1b46da@oss.nttdata.com

doc/src/sgml/ref/create_index.sgml

@@ -402,8 +402,7 @@ CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ [ IF NOT EXISTS ] <replaceable class=
    <varlistentry id="index-reloption-deduplication" xreflabel="deduplicate_items">
     <term><literal>deduplicate_items</literal>
      <indexterm>
-      <primary><varname>deduplicate_items</varname></primary>
-      <secondary>storage parameter</secondary>
+      <primary><varname>deduplicate_items</varname> storage parameter</primary>
      </indexterm>
     </term>
     <listitem>