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

@@ -28,12 +28,12 @@ dataset_info:
     dtype: string
   config_name: penn_treebank
   splits:
-  - name: test
-    num_bytes: 453710
-    num_examples: 3761
   - name: train
     num_bytes: 5143706
     num_examples: 42068
+  - name: test
+    num_bytes: 453710
+    num_examples: 3761
   - name: validation
     num_bytes: 403156
     num_examples: 3370