Reorder split names

Reorder split names in the YAML metadata to be aligned with the order in the loading script.
See: https://github.com/huggingface/datasets/issues/5258

README.md

@@ -157,12 +157,12 @@ dataset_info:
       - name: child_id
         dtype: int32
   splits:
-  - name: train
-    num_bytes: 1135684803
-    num_examples: 200000
   - name: validation
     num_bytes: 55823979
     num_examples: 10000
+  - name: train
+    num_bytes: 1135684803
+    num_examples: 200000
   download_size: 259652560
   dataset_size: 1191508782
 ---