acme/autocert: include email in example

At Let's Encrypt, we've found that most autocert users do not provide an
email address, which makes it hard to get in touch when things go wrong
with their client. Demonstrating how to provide an email will probably
encourage more people to provide one.

acme/autocert/example_test.go

@@ -24,6 +24,7 @@ func ExampleManager() {
 	m := &autocert.Manager{
 		Cache:      autocert.DirCache("secret-dir"),
 		Prompt:     autocert.AcceptTOS,
+		Email:      "[email protected]",
 		HostPolicy: autocert.HostWhitelist("example.org", "www.example.org"),
 	}
 	s := &http.Server{