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

@@ -29,9 +29,9 @@ dataset_info:
     sequence:
       class_label:
         names:
-          0: O
-          1: B
-          2: I
+          '0': O
+          '1': B
+          '2': I
   config_name: linnaeus
   splits:
   - name: train