Update CHANGELOG for v1.5.7.3

***NO_CI***

Author: nfbot

CHANGELOG.md

@@ -1,6 +1,8 @@
 # Changelog
 
-[Full Changelog](https://github.com/nanoframework/nanoFramework.Networking.Sntp/compare/v1.5.7-preview.5...HEAD)
+## [v1.5.7-preview.7](https://github.com/nanoframework/nanoFramework.Networking.Sntp/tree/v1.5.7-preview.7) (2021-12-02)
+
+[Full Changelog](https://github.com/nanoframework/nanoFramework.Networking.Sntp/compare/v1.5.7-preview.5...v1.5.7-preview.7)
 
 **Documentation and other chores:**