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

@@ -27,16 +27,16 @@ dataset_info:
     dtype:
       class_label:
         names:
-          0: T - shirt / top
-          1: Trouser
-          2: Pullover
-          3: Dress
-          4: Coat
-          5: Sandal
-          6: Shirt
-          7: Sneaker
-          8: Bag
-          9: Ankle boot
+          '0': T - shirt / top
+          '1': Trouser
+          '2': Pullover
+          '3': Dress
+          '4': Coat
+          '5': Sandal
+          '6': Shirt
+          '7': Sneaker
+          '8': Bag
+          '9': Ankle boot
   config_name: fashion_mnist
   splits:
   - name: train