BSD 3 license (and maybe others) require a copyright notice at the top of the license file

While looking at [the typical BSD-3 clause license](https://opensource.org/license/bsd-3-clause), you can see the `copyright` line at the top that is expected to be filled in.

Later on in the first clause of the license, it mentions this copyright notice.

`ament_copyright` today doesn't require a BSD-3 License file to have a copyright notice at the top, but I think it should.