Fix plpgsql "PERFORM" markup.

Backpatch to 9.0 and 9.1.

Author: bmomjian

doc/src/sgml/plpgsql.sgml

@@ -940,7 +940,7 @@ PERFORM <replaceable>query</replaceable>;
      result.  Write the <replaceable>query</replaceable> the same
      way you would write an SQL <command>SELECT</> command, but replace the
      initial keyword <command>SELECT</> with <command>PERFORM</command>.
-     For <keyword>WITH</> queries, use <keyword>PERFORM</> and then
+     For <command>WITH</> queries, use <command>PERFORM</> and then
      place the query in parentheses.  (In this case, the query can only
      return one row.)
      <application>PL/pgSQL</application> variables will be