Update release date in Changelog.md

Summary: Needed some more time to resolve some packaging issues with conda.

Reviewed By: qingfeng10

Differential Revision: D23672868

fbshipit-source-id: 5d363f1f7c4d7c2e1084d1951b5d5c01a78b8e93

Author: Balandat

CHANGELOG.md

@@ -3,7 +3,7 @@
 The release log for BoTorch.
 
 
-## [0.3.1] - Sep 12, 2020
+## [0.3.1] - Sep 15, 2020
 
 Maintenance Release