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

@@ -30,9 +30,9 @@ dataset_info:
       dtype:
         class_label:
           names:
-            0: hatespeech
-            1: normal
-            2: offensive
+            '0': hatespeech
+            '1': normal
+            '2': offensive
     - name: annotator_id
       dtype: int32
     - name: target