docs: Fix bit_count example output

The returnvalue for the bit_count(::bytea) example was assuming a
non-default value of standard_conforming_strings.  This was fixed
in the tests in commit ebedd0c7.

Author: wangzk.fnstxz@fujitsu.com
Discussion: https://postgr.es/m/OSZPR01MB6551FFAC1088C82C3D799BE0FAEB9@OSZPR01MB6551.jpnprd01.prod.outlook.com
Backpatch-through: 14

doc/src/sgml/func.sgml

@@ -4085,7 +4085,7 @@ SELECT format('Testing %3$s, %2$s, %s', 'one', 'two', 'three');
        </para>
        <para>
         <literal>bit_count('\x1234567890'::bytea)</literal>
-        <returnvalue>31</returnvalue>
+        <returnvalue>15</returnvalue>
        </para></entry>
       </row>