Change license from CC-BY-4.0 to CC-BY-SA-4.0

Change the licence from CC-BY-4.0 to CC-BY-SA-4.0 at the suggestion of the IPR team, as the dataset contains text fragments extracted from Wikipedia.

README.md

@@ -1,5 +1,5 @@
 ---
-license: cc-by-4.0
+license: cc-by-sa-4.0
 task_categories:
 - question-answering
 - summarization