doc: a small improvement about pg_am description

Hi,

When reading the documentation about operator class, I found
the following description:

The pg_am table contains one row for every index access method.
Support for access to regular tables is built into PostgreSQL,
but all index access methods are described in pg_am.

It seems to me that this description says pg_am contains only
index access methods but not table methods. I wonder it is missed
to fix this when tableam was supported and other documentation
was changed in b73c3a11963c8bb783993cfffabb09f558f86e37.

Attached is a patch to remove the sentence that starts with
"Support for access to regular tables is ....".

Ragards,
Yugo Nagata

--
Yugo NAGATA <nagata(at)sraoss(dot)co(dot)jp>