Remove 'ZFS on Linux' references from PR Template

As mentioned in the #OpenZFS IRC channel (thanks "Toomas Soome"):
The OpenZFS PR Template still mentions "ZFS on Linux".
This changes that reference and updates the URLs.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Garrett Fields <ghfields@gmail.com>
Closes #10868