Replace YAML keys from int to str

Replace YAML metadata integer keys with strings, as the Hub does not support integers.
See: https://github.com/huggingface/datasets/issues/5275

README.md

@@ -39,8 +39,8 @@ dataset_info:
     dtype:
       class_label:
         names:
-          0: neg
-          1: pos
+          '0': neg
+          '1': pos
   splits:
   - name: train
     num_bytes: 22290345