Fix grammar

backpatched to 8.1

doc/src/sgml/perform.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/perform.sgml,v 1.83 2010/07/13 20:57:19 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/perform.sgml,v 1.84 2010/07/26 20:28:49 petere Exp $ -->
 
  <chapter id="performance-tips">
   <title>Performance Tips</title>
@@ -1086,7 +1086,7 @@ SELECT * FROM x, y, a, b, c WHERE something AND somethingelse;
       <para>
        You can get the effect of disabling foreign keys by using
        the <option>--disable-triggers</> option &mdash; but realize that
-       that eliminates, rather than just postponing, foreign key
+       that eliminates, rather than just postpones, foreign key
        validation, and so it is possible to insert bad data if you use it.
       </para>
      </footnote>