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 openzfs#11918

man/man8/zfsprops.8

@@ -1169,7 +1169,7 @@ will be allocated in the special class.
 .Pp
 Before setting this property, a special class vdev must be added to the
 pool. See
-.Xr zpool 8
+.Xr zpoolconcepts 8
 for more details on the special allocation class.
 .It Sy mountpoint Ns = Ns Pa path Ns | Ns Sy none Ns | Ns Sy legacy
 Controls the mount point used for this file system.