doc: PG 13 relnotes: fix link for grouping sets hash overflow

Use "guc-enable-groupingsets-hash-disk".

Reported-by: TAKATSUKA Haruka

Discussion: https://postgr.es/m/16468-7939d39f1786516c@postgresql.org

Backpatch-through: master

Author: bmomjian

doc/src/sgml/release-13.sgml

@@ -645,7 +645,7 @@ Author: Jeff Davis <jdavis@postgresql.org>
        <para>
         Previously, hash aggregation was avoided if it was expected to use
         more than <xref linkend="guc-work-mem"/> memory.  This is controlled
-        by <xref linkend="guc-enable-hashagg-disk"/>.
+        by <xref linkend="guc-enable-groupingsets-hash-disk"/>.
        </para>
       </listitem>