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

@@ -32,9 +32,9 @@ dataset_info:
     dtype:
       class_label:
         names:
-          0: entailment
-          1: neutral
-          2: contradiction
+          '0': entailment
+          '1': neutral
+          '2': contradiction
   - name: relatedness_score
     dtype: float32
   - name: entailment_AB