Update readme.txt

Author: ataxali

data/volume/readme.txt

@@ -1,5 +1,5 @@
 Due to github's file size limits, the main volume-data tarfile is hosted on box, please download 'volume_connectomes.tar.gz' (filesize = 2.35 gb). using the following link:
-https://umich.box.com/s/skhmo7twdtavdagm7h5kr93oc33sc1by
+https://www.dropbox.com/s/mb5hd0s9lmnc5gr/volume_connectomes.tar.gz?dl=0
 
 The tarfile "volume_connectomes.tar.gz" contains 12 .pickle files. 
 One pickle file for each of the following combinations - {15, 7, 3 minutes} x {day 1, 2} x {5, 2 fd thresholding}