BUG #4873: docs: no link from dml-update to sql-update

The following bug has been logged online:

Bug reference:      4873
Logged by:          Brad Bowman
Email address:      postgres@bereft.net
PostgreSQL version: 8.3
Operating system:   n/a
Description:        docs: no link from dml-update to sql-update
Details:

There seems to be an oversight in the dml-update
document.

http://www.postgresql.org/docs/8.3/interactive/dml-update.html

  doesn't link to

http://www.postgresql.org/docs/8.3/interactive/sql-update.html

while both dml-insert and dml-delete do link to their
respective pages.