Doc: Fixed the result of the bit_count example

Hello:

There is a small problem with the documentation for the previously added SQL function “bit_count”.

In the doc, (https://www.postgresql.org/docs/14/functions-binarystring.html)

“bit_count('\x1234567890'::bytea)” result is "31" , but the actual result is "15".

Similar problems have been fixed in test file (commit: ebedd0c78fc51c293abe56e99a18c67af14da0c9),  but the doc has not been modified.

Regards,

Wang