update link to pkg.go.dev

README.md

@@ -4,7 +4,7 @@ HTTP Hashcash implemented in Go.
 
 Explanation at https://therootcompany.com/blog/http-hashcash/
 
-Go docs at https://godoc.org/git.rootprojects.org/root/hashcash
+Go API docs at https://pkg.go.dev/git.rootprojects.org/root/hashcash?tab=doc
 
 # CLI Usage