Change pg_attribute textual link to an actual link

In the System catalog/pg_attrdef documentation, there is a reference to
pg_attribute with the text “see below”.  That makes sense in the PDF version
(and similar renderings), but in the web version it seems a bit odd as
pg_attribute is on the next page, not below.

The attached patch changes it to an actual link instead, but simply removing
the “see below” bit is another approach which isn’t out of line with other
parts of the documentation (I opted for link since I personally prefer that).

cheers ./daniel