Update README.md

update code to consider subset rather than split

README.md

@@ -282,7 +282,7 @@ from datasets import load_dataset
 dataset = load_dataset('LeMaterial/LeMat-Bulk', 'compatible_pbe')
 
 # convert to Pandas, if you prefer working with this type of object:
-df = dataset['compatible_pbe'].to_pandas()
+df = dataset['train'].to_pandas()
 ```
 
 ## Data fields