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

@@ -7,12 +7,12 @@ dataset_info:
     dtype: string
   config_name: plain_text
   splits:
-  - name: test
-    num_bytes: 42942045
-    num_examples: 306688
   - name: train
     num_bytes: 4238206516
     num_examples: 30301028
+  - name: test
+    num_bytes: 42942045
+    num_examples: 306688
   download_size: 1792209805
   dataset_size: 4281148561
 ---