Update release date

According to microsoft/TypeScript#38510 (comment)

Author: Lorenz Henk

packages/typescriptlang-org/src/lib/release-plan.json

@@ -5,5 +5,5 @@
   "last_release_date": "05/12/2020",
   "upcoming_beta_date": "06/23/2020",
   "upcoming_rc_date": "08/04/2020",
-  "upcoming_release_date": "08/18/2020"
+  "upcoming_release_date": "08/20/2020"
 }