Update CHANGELOG.md link (rapidsai#3956)

Updated the link to access CHANGELOG.md for branch-21.08 as the current link throws 404

Authors:
  - https://github.com/Salonijain27

Approvers:
  - Dante Gama Dessavre (https://github.com/dantegd)

URL: rapidsai#3956

CHANGELOG.md

@@ -1,6 +1,6 @@
 # cuML 21.08.00 (Date TBD)
 
-Please see https://github.com/rapidsai/cuml/releases/tag/v21.08.0a for the latest changes to this development branch.
+Please see https://github.com/rapidsai/cuml/releases/tag/v21.08.00a for the latest changes to this development branch.
 
 # cuML 21.06.00 (Date TBD)