Rename SendMail PLAIN auth example

To make go vet happy:

    ./example_test.go:60:1: ExampleSendMail_PlainAuth refers to unknown field or method: SendMail.PlainAuth

example_test.go

@@ -57,7 +57,7 @@ var (
 	recipients = []string{"[email protected]"}
 )
 
-func ExampleSendMail_PlainAuth() {
+func ExampleSendMail_plainAuth() {
 	// hostname is used by PlainAuth to validate the TLS certificate.
 	hostname := "mail.example.com"
 	auth := sasl.NewPlainClient("", "[email protected]", "password")