Doc: remove extra comma in syntax summary for array_fill().

Noted by Scott Ure.  Back-patch to all supported branches.

Discussion: https://postgr.es/m/152199346794.4544.1888397173908716912@wrigleys.postgresql.org

doc/src/sgml/func.sgml

@@ -12761,7 +12761,7 @@ SELECT NULLIF(value, '(none)') ...
        <row>
         <entry>
          <literal>
-          <function>array_fill</function>(<type>anyelement</type>, <type>int[]</type>,
+          <function>array_fill</function>(<type>anyelement</type>, <type>int[]</type>
           <optional>, <type>int[]</type></optional>)
          </literal>
         </entry>