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.

Change-Id: I0aa1ecea7cfbe55ae155804194fb57a8e695c658
GitHub-Last-Rev: 902b5fe2682a6be5af91e886272fd680fd67a94e
GitHub-Pull-Request: golang/crypto#186
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/323313
Reviewed-by: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
Trust: Katie Hockman <katie@golang.org>

acme/autocert/example_test.go

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