Fix the default checksum algorithm in the manpage

The manpage reports fletcher2, but in zio.h ZIO_CHECKSUM_ON_VALUE
is defined to ZIO_CHECKSUM_FLETCHER_4.

Signed-off-by: Massimo Maggi <me@massimo-maggi.eu>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1628