zfs_get: change time format string from %k to %H

zfs_get: change time format string from %k to %H

Issue #10090 reported that snapshots created between midnight and 1 AM
are missing a padded zero in the creation property

This change fixes the bug reported in issue #10090 where snapshots
created between midnight and 1 AM were missing a padded zero in the
creation timestamp output.

The leading zero was missing because the time format string used `%k`
which formats the hour as a decimal number from 0 to 23 where single
digits are preceded by blanks[0] and is fixed by changing it to `%H`
which formats the hour as 00-23.

The difference in output is as below

```
-Thu Mar 26  0:39 2020
+Thu Mar 26 00:39 2020
```

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Alex John <alex@stty.io>
Closes #10090
Closes #10153