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

@@ -40,9 +40,9 @@ dataset_info:
     sequence:
       class_label:
         names:
-          0: y
-          1: n
-          2: x
+          '0': y
+          '1': n
+          '2': x
   - name: answers
     sequence:
     - name: texts