update max_variance limit in zdb_block_size_histogram test for CI

Commit 2d7843401a628ef8c483229742dd58bca70bc27e had previously
updated this hardcoded limit to allow for CI testing. As there
is no deterministic pass/fail value, the need has arisen for
one more small increase.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Edmund Nadolski <edmund.nadolski@ixsystems.com>
Closes #15252