BUG #18543: Mistake in docs example

The following bug has been logged on the website:

Bug reference: 18543
Logged by: Ondřej Navrátil
Email address: onavratil@monetplus.cz
PostgreSQL version: 16.3
Operating system: windows
Description:

https://www.postgresql.org/docs/17/ddl-identity-columns.html

The second example:

```
CREATE TABLE people (
id bigint GENERATED BY DEFAULT IDENTITY,
...,
);
```

is missing **AS**, correct version:

```
CREATE TABLE people (
id bigint GENERATED BY DEFAULT AS IDENTITY,
...,
);
```

... as per syntax here
https://www.postgresql.org/docs/current/sql-createtable.html
```
...
GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( sequence_options ) ] |
...
```