Update README.md

Make the environment variable compatible with dolma.py

README.md

@@ -108,7 +108,7 @@ Then, to load this data using HuggingFace's `datasets` library, you can use the
 import os
 from datasets import load_dataset
 
-os.environ["DATA_DIR"] = "<path_to_your_data_directory>"
+os.environ["DOLMA_DATA_DIR"] = "<path_to_your_data_directory>"
 dataset = load_dataset("allenai/dolma", split="train")
 ```