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

@@ -63,12 +63,12 @@ dataset_info:
   - name: wikiDocumentIdx
     dtype: int64
   splits:
-  - name: test
-    num_bytes: 379521
-    num_examples: 960
   - name: train
     num_bytes: 3142398
     num_examples: 8060
+  - name: test
+    num_bytes: 379521
+    num_examples: 960
   - name: validation
     num_bytes: 368726
     num_examples: 942