Update copyright year and license reference in the static-site README

The year and filename were long out of date, which I noticed when
putting a LICENSE notice in the root of the repo.

static-site/README.md

@@ -83,6 +83,6 @@ The static documentation is automatically rebuilt every time the (parent) repo i
 
 ## License and copyright
 
-Copyright 2014-2018 Trevor Bedford and Richard Neher.
+Copyright 2014-2021 Trevor Bedford and Richard Neher.
 
-Source code to Nextstrain is made available under the terms of the [GNU Affero General Public License](LICENSE.txt) (AGPL). Nextstrain is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for more details.
+Source code to Nextstrain is made available under the terms of the [GNU Affero General Public License 3.0](../LICENSE) (AGPL-3.0). Nextstrain is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for more details.