Fix xreflabel for hot_standby_feedback.

Rather remarkable that this has been wrong since 9.1 and nobody noticed.

doc/src/sgml/config.sgml

@@ -2778,7 +2778,7 @@ include_dir 'conf.d'
       </listitem>
      </varlistentry>
 
-     <varlistentry id="guc-hot-standby-feedback" xreflabel="hot_standby">
+     <varlistentry id="guc-hot-standby-feedback" xreflabel="hot_standby_feedback">
       <term><varname>hot_standby_feedback</varname> (<type>boolean</type>)
       <indexterm>
        <primary><varname>hot_standby_feedback</> configuration parameter</primary>