doc: PG 13 relnotes: update bool_plperl item

Reported-by: Daniel Gustafsson

Discussion: https://postgr.es/m/54F7560D-498A-4E51-BAA4-17D4AAB2AA57@yesql.se

Backpatch-through: master

Author: bmomjian

doc/src/sgml/release-13.sgml

@@ -2890,9 +2890,10 @@ Author: Tom Lane <tgl@sss.pgh.pa.us>
 -->
 
       <para>
-       Add extension <application>bool_plperl</application> which transforms
-       <acronym>SQL</acronym> booleans to/from PL/Perl booleans (Ivan
-       Panchenko) WHERE IS THIS DOCUMENTED?
+       Add extension <link
+       linkend="plperl"><application>bool_plperl</application></link>
+       which transforms <acronym>SQL</acronym> booleans to/from PL/Perl
+       booleans (Ivan Panchenko)
       </para>
      </listitem>