update README

[skip ci]

Author: doghappy

README.md

@@ -15,7 +15,7 @@ An elegant socket.io client for .NET, Supports `.NET Standard 2.0`, support sock
   - [ClientWebSocket Options](#clientwebsocket-options)
   - [Windows 7 Support](#windows-7-support)
 - [Breaking changes](#breaking-changes)
-  - [Breaking changes in 3.x](#breaking-changes-in-3x)
+  - [Breaking changes in 3.0.0](#breaking-changes-in-300)
   - [Breaking changes in 2.2.4](#breaking-changes-in-224)
   - [Breaking changes in 2.2.0](#breaking-changes-in-220)
 - [Change log](#change-log)