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

@@ -78,9 +78,9 @@ dataset_info:
     dtype:
       class_label:
         names:
-          0: equal
-          1: sentence1
-          2: sentence2
+          '0': equal
+          '1': sentence1
+          '2': sentence2
   splits:
   - name: train
     num_bytes: 2102667