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

@@ -27,22 +27,22 @@ dataset_info:
     dtype:
       class_label:
         names:
-          0: letter
-          1: form
-          2: email
-          3: handwritten
-          4: advertisement
-          5: scientific report
-          6: scientific publication
-          7: specification
-          8: file folder
-          9: news article
-          10: budget
-          11: invoice
-          12: presentation
-          13: questionnaire
-          14: resume
-          15: memo
+          '0': letter
+          '1': form
+          '2': email
+          '3': handwritten
+          '4': advertisement
+          '5': scientific report
+          '6': scientific publication
+          '7': specification
+          '8': file folder
+          '9': news article
+          '10': budget
+          '11': invoice
+          '12': presentation
+          '13': questionnaire
+          '14': resume
+          '15': memo
   splits:
   - name: train
     num_bytes: 38816373360