Update stackexchange license (#4842)

The correct license of the stackexchange subset of the Pile is `cc-by-sa-4.0`, as can for example be seen here: https://stackoverflow.com/help/licensing

Commit from https://github.com/huggingface/datasets/commit/1aaf2f308a2d56ede6cde3606d19ce1f1e689132

README.md

@@ -6,7 +6,7 @@ language_creators:
 language:
 - en
 license:
-- mit
+- cc-by-sa-4.0
 multilinguality:
 - monolingual
 pretty_name: Stack Exchange
@@ -165,4 +165,4 @@ The dataset is in English.
 
 ### Contributions
 Thanks to [sdtblck](https://github.com/sdtblck) for creating the dataset.
-Thanks to [richarddwang](https://github.com/richarddwang) for adding the dataset.
+Thanks to [richarddwang](https://github.com/richarddwang) for adding the dataset.