Update readme.txt

- Fix API URL.

***NO_CI***

Author: josesimoes

assets/readme.txt

@@ -6,7 +6,7 @@
 
 ===================================================================================
 
-API docs: https://docs.nanoframework.net/api/nanoFramework.Json.html
+API docs: https://docs.nanoframework.net/api/nanoFramework.MagicBit.html
 
 Browse our samples repository: https://github.com/nanoframework/samples