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

@@ -35,8 +35,8 @@ dataset_info:
     dtype:
       class_label:
         names:
-          0: '0'
-          1: '1'
+          '0': '0'
+          '1': '1'
   config_name: plain_text
   splits:
   - name: test