Doc: fix bogus example.

This wasn't incorrect SQL, but it was doing cm-to-inch conversion
backward, so it might confuse readers.

Per bug #15849 from TAKATSUKA Haruka.

Discussion: https://postgr.es/m/15849-37ad0c561a836107@postgresql.org

doc/src/sgml/ddl.sgml

@@ -259,7 +259,7 @@ CREATE TABLE products (
 CREATE TABLE people (
     ...,
     height_cm numeric,
-    height_in numeric <emphasis>GENERATED ALWAYS AS (height_cm * 2.54) STORED</emphasis>
+    height_in numeric <emphasis>GENERATED ALWAYS AS (height_cm / 2.54) STORED</emphasis>
 );
 </programlisting>
    The keyword <literal>STORED</literal> must be specified to choose the