Update README.md

The original repo (listed in this README) by the paper's authors states the license to be Apache 2.0: https://github.com/afshinrahimi/mmner/commit/35f12bf568f1b44ab4630a73908e97ddd0af4d71

Adding this info as a metadata in the README.md makes the information more easily findable for anyone looking to use the dataset.

README.md

@@ -178,7 +178,7 @@ language:
 - zea
 - zh
 license:
-- unknown
+- apache-2.0
 multilinguality:
 - multilingual
 size_categories: