Update curl commands on get.zfsbootmenu.org

[timkgh]

First and second curl commands should also have -LJO flags:

Release and recovery images

curl https://get.zfsbootmenu.org/:asset/:build
...

Other assets

curl https://get.zfsbootmenu.org/:asset

[zdykstra]

Additional flags aren't provided for those commands because they're generic examples indicating how to access files.

[timkgh]

It might be confusing because without -L the curl command appears to do nothing. Perhaps consider replacing curl with GET or Fetch or something that is not a command then.

[zdykstra]

Yeah, that helps clarify it. I've swapped GET in for curl for the first two usage stanzas.