Fix the documentation about session_preload_libraries.

[japinli]

Here is a sentence about session_preload_libraries in the PostgreSQL documentation:

Only superusers and users with the appropriate SET privilege can change this setting.

For a non-superuser without SET privilege, the ALTER DATABASE <dbname> SET session_preload_libraries = ... will cause a permission denied.

postgres=# CREATE USER u01;
CREATE ROLE
postgres=# CREATE DATABASE db01 OWNER u01;
CREATE DATABASE
postgres=# \c db01 u01
You are now connected to database "db01" as user "u01".
db01=> DO $$
BEGIN
    EXECUTE format('ALTER DATABASE %I SET session_preload_libraries = ''$libdir/plugins/pgtt''', current_database());
END
$$;
ERROR:  permission denied to set parameter "session_preload_libraries"
CONTEXT:  SQL statement "ALTER DATABASE db01 SET session_preload_libraries = '$libdir/plugins/pgtt'"
PL/pgSQL function inline_code_block line 3 at EXECUTE

This PR also removes the LOAD '$libdir/plugins/pgtt'; command in the example, because we use session_preload_libraries to load this library.

If we want to keep this command, we should revert the commit ef79a91, since we should link the library into $libdir/plugins.