mount_nfs.8: Remove reference to NFSv4 minor version 2

The MFC of commit aba7a81ab71e was not accurate, since stable/12 does
not support NFSv4.2. This patch deletes the reference to minor
version 2.

Since this is a minor change and the MFC of another mount_nfs.8
is coming soon and will change the date, this patch does not
change the date.

It is a minor content change.

It is a direct commit to stable/12.