fix typo in readme

Author: zfletch

README.md

@@ -140,4 +140,4 @@ For instructions on how to make your trees available in the [Alpheios Reading To
 ## Licenses
 
 The code is licensed under the MIT license (see `LICENSE` file).
-The treebanks are licensed under the CC0 1.0 license (see `TREEBANK_LICENSE` file).
+The treebanks are licensed under the CC BY-SA 4.0 license (see `TREEBANK_LICENSE` file).