Doc: fix documentation example for bytea hex output format.

Per report from rsindlin

Discussion: https://postgr.es/m/167907221210.1803488.5939223864945604536@wrigleys.postgresql.org

doc/src/sgml/datatype.sgml

@@ -1438,7 +1438,12 @@ SELECT b, char_length(b) FROM test2;
    <para>
     Example:
 <programlisting>
-SELECT '\xDEADBEEF';
+SET bytea_output = 'hex';
+
+SELECT '\xDEADBEEF'::bytea;
+   bytea
+------------
+ \xdeadbeef
 </programlisting>
    </para>
   </sect2>