docs: Fix small copy & paste mistake.

Author: Justin Pryzby
Discussion: https://postgr.es/m/20190418005115.r4mat75wvlski3ij@alap3.anarazel.de

doc/src/sgml/ref/insert.sgml

@@ -554,7 +554,7 @@ INSERT <replaceable>oid</replaceable> <replaceable class="parameter">count</repl
    The <replaceable class="parameter">count</replaceable> is the number of
    rows inserted or updated.  <replaceable>oid</replaceable> is always 0 (it
    used to be the <acronym>OID</acronym> assigned to the inserted row if
-   <replaceable>rows</replaceable> was exactly one and the target table was
+   <replaceable>count</replaceable> was exactly one and the target table was
    declared <literal>WITH OIDS</literal> and 0 otherwise, but creating a table
    <literal>WITH OIDS</literal> is not supported anymore).
   </para>