Fix file path for tar command

Author: matthew-brett

README.md

@@ -13,7 +13,7 @@ This README file has instructions on how to get, validate and process the data.
 ```
 cd data
 curl -L https://figshare.com/ndownloader/files/{DATA_ID} -o group_data.tar
-tar zxvf group.tar
+tar zxvf group_data.tar
 cd ..
 ```