readme: update example link to point to pkg.go.dev

Author: tulir

README.md

@@ -14,7 +14,7 @@ discussions.
 
 ## Usage
 The [godoc](https://pkg.go.dev/go.mau.fi/whatsmeow) includes docs for all methods and event types.
-There's also a [simple example](https://godocs.io/go.mau.fi/whatsmeow#example-package) at the top.
+There's also a [simple example](https://pkg.go.dev/go.mau.fi/whatsmeow#example-package) at the top.
 
 ## Features
 Most core features are already present: