Doc: normalize vacuum_multixact_failsafe_age ID.

Author: Pavel Luzanov <p.luzanov@postgrespro.ru>
Discussion: https://postgr.es/m/c71a3cfc-a267-3d9f-1b44-fbd668d0ab10@postgrespro.ru
Backpatch: 14-, where the failsafe was introduced.

doc/src/sgml/config.sgml

@@ -8795,7 +8795,7 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
       </listitem>
      </varlistentry>
 
-     <varlistentry id="guc-multixact-failsafe-age" xreflabel="vacuum_multixact_failsafe_age">
+     <varlistentry id="guc-vacuum-multixact-failsafe-age" xreflabel="vacuum_multixact_failsafe_age">
       <term><varname>vacuum_multixact_failsafe_age</varname> (<type>integer</type>)
       <indexterm>
        <primary><varname>vacuum_multixact_failsafe_age</varname> configuration parameter</primary>

doc/src/sgml/release-14.sgml

@@ -911,7 +911,7 @@ Author: Peter Geoghegan <pg@bowt.ie>
 
        <para>
         This is controlled by <xref linkend="guc-vacuum-failsafe-age"/>
-        and <xref linkend="guc-multixact-failsafe-age"/>.
+        and <xref linkend="guc-vacuum-multixact-failsafe-age"/>.
        </para>
       </listitem>