doc: Fix INSERT statement syntax for identity columns

The INSERT statements in the examples were erroneously using
VALUE instead of VALUES. Backpatch to v17 where the examples
were added through a37bb7c139.

Reported-by: shixiong327926@gmail.com
Discussion: https://postgr.es/m/172958472112.696.6075270400394560263@wrigleys.postgresql.org
Backpatch-through: 17

doc/src/sgml/ddl.sgml

@@ -271,8 +271,8 @@ CREATE TABLE people (
    example, with the above definitions and assuming additional appropriate
    columns, writing
 <programlisting>
-INSERT INTO people (name, address) VALUE ('A', 'foo');
+INSERT INTO people (name, address) VALUES ('A', 'foo');
-INSERT INTO people (name, address) VALUE ('B', 'bar');
+INSERT INTO people (name, address) VALUES ('B', 'bar');
 </programlisting>
    would generate values for the <literal>id</literal> column starting at 1
    and result in the following table data:
@@ -285,7 +285,7 @@ INSERT INTO people (name, address) VALUE ('B', 'bar');
    Alternatively, the keyword <literal>DEFAULT</literal> can be specified in
    place of a value to explicitly request the sequence-generated value, like
 <programlisting>
-INSERT INTO people (id, name, address) VALUE (<emphasis>DEFAULT</emphasis>, 'C', 'baz');
+INSERT INTO people (id, name, address) VALUES (<emphasis>DEFAULT</emphasis>, 'C', 'baz');
 </programlisting>
    Similarly, the keyword <literal>DEFAULT</literal> can be used in
    <command>UPDATE</command> commands.