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

@@ -9,7 +9,6 @@ license:
 - cc-by-sa-3.0
 multilinguality:
 - monolingual
-pretty_name: Natural Questions
 size_categories:
 - 100K<n<1M
 source_datasets:
@@ -19,6 +18,7 @@ task_categories:
 task_ids:
 - open-domain-qa
 paperswithcode_id: natural-questions
+pretty_name: Natural Questions
 dataset_info:
   features:
   - name: id
@@ -73,8 +73,8 @@ dataset_info:
       dtype:
         class_label:
           names:
-            0: 'NO'
-            1: 'YES'
+            '0': 'NO'
+            '1': 'YES'
     - name: long_answer_candidates
       sequence:
       - name: start_token