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

@@ -80,12 +80,12 @@ dataset_info:
           1: pos
   config_name: plain_text
   splits:
-  - name: test
-    num_bytes: 32650697
-    num_examples: 25000
   - name: train
     num_bytes: 33432835
     num_examples: 25000
+  - name: test
+    num_bytes: 32650697
+    num_examples: 25000
   - name: unsupervised
     num_bytes: 67106814
     num_examples: 50000