Table AM callbacks referring to heap in declarations (+typos)

Hi all,

While going through the table AM callbacks, I have bumped into a
couple of references to heap.  I think that we should make that more
generic by using the term "table" as done when opening relations and
such.  Attached is a cleanup patch.

While on it, I found a set of typos which looked like a copy-pasto
which got spread => "index_nfo".  I know, these are nits, but I think
that this also reduces the confusion with the way table AM callbacks
are presented to extension developers.

Thanks,
--
Michael