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@sraoss.co.jp>