man.8: Add bookmark to list of types

While checking bash_completion I missed bookmark as type.

```
# zfs get type zpool2#b
NAME      PROPERTY  VALUE     SOURCE
zpool2#b  type      bookmark  -
```

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Grischa Zengel <github.zfsonlinux@zengel.info>
Closes openzfs#10419

man/man8/zfsprops.8

@@ -282,8 +282,9 @@ has been set somewhere in the tree under which the dataset resides.
 The type of dataset:
 .Sy filesystem ,
 .Sy volume ,
+.Sy snapshot ,
 or
-.Sy snapshot .
+.Sy bookmark .
 .It Sy used
 The amount of space consumed by this dataset and all its descendents.
 This is the value that is checked against this dataset's quota and reservation.