Fixed incorrect man page reference in zfsprops(8)

The special_small_blocks section directed readers to zpool(8) for
documentation on special allocation classes, while they are actually
documented in zpoolconcepts(8).

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Daniel Stevenson <daniel@dstev.net>
Closes #11918