ALTER TABLE ... CLUSTER ON synopsis

Hi all,

The reference page for ALTER TABLE[1] correctly has the synopsis:

    CLUSTER ON <replaceable class="PARAMETER">index_name</replaceable>

but later leaves off the "ON" keyword:

    <term><literal>CLUSTER</literal></term>

I think the latter description should match the former, especially
since 'ON' is mandatory; small patch attached.

Josh

[1] http://www.postgresql.org/docs/devel/static/sql-altertable.html