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

@@ -29,8 +29,8 @@ dataset_info:
     dtype:
       class_label:
         names:
-          0: '1'
-          1: '-1'
+          '0': '1'
+          '1': '-1'
   splits:
   - name: train
     num_bytes: 1020146