Update Changelog Link (rapidsai#7550)

The tag used for pre-releases was recently changed, so this PR updates the link in the changelog.

Authors:
  - AJ Schmidt (@ajschmidt8)

Approvers:
  - Jordan Jacobelli (@Ethyling)

URL: rapidsai#7550

CHANGELOG.md

@@ -1,6 +1,6 @@
 # cuDF 0.19.0 (Date TBD)
 
-Please see https://github.com/rapidsai/cudf/releases/tag/branch-0.19-latest for the latest changes to this development branch.
+Please see https://github.com/rapidsai/cudf/releases/tag/v0.19.0a for the latest changes to this development branch.
 
 # cuDF 0.18.0 (24 Feb 2021)