Update CHANGELOG for v1.2.1

***NO_CI***

Author: nfbot

CHANGELOG.md

@@ -8,6 +8,10 @@
 
 - Remove CultureInfo unused field [\#72](https://github.com/nanoframework/lib-CoreLibrary/pull/72)
 
+**Fixed bugs:**
+
+- Fix SortableDateTimePattern [\#73](https://github.com/nanoframework/lib-CoreLibrary/pull/73)
+
 ## [v1.2.0](https://github.com/nanoframework/lib-CoreLibrary/tree/v1.2.0) (2019-04-23)
 [Full Changelog](https://github.com/nanoframework/lib-CoreLibrary/compare/v1.1.2...v1.2.0)