docs(transport-smtp): fix docs for domain field

The `domain` field does not get “send to the server”,
but is used to check the authenticity of the server.

lettre/src/smtp/client/net.rs

@@ -12,7 +12,7 @@ use std::time::Duration;
 pub struct ClientTlsParameters {
     /// A connector from `native-tls`
     pub connector: TlsConnector,
-    /// The domain to send during the TLS handshake
+    /// The domain name which is expected in the TLS certificate from the server
     pub domain: String,
 }