Doc: fix minor release-note typo.

Liudmila Mantrova

Discussion: https://postgr.es/m/d13458be-c4b9-0fd8-f333-c8de4d0c9120@postgrespro.ru

doc/src/sgml/release-10.sgml

@@ -469,7 +469,7 @@ Branch: REL9_5_STABLE [e4ff71158] 2018-03-28 13:26:43 -0400
 -->
      <para>
       Fix executor crash due to double free in some <literal>GROUPING
-      SET</literal> usages (Peter Geoghegan)
+      SETS</literal> usages (Peter Geoghegan)
      </para>
     </listitem>