Fix SmtpServerOptionsBuilder naming for CustomSmtpGreeting

[tinohager]

The change renames the CustomGreetingMessage method to CustomSmtpGreeting and updates the example delegate parameter to use a more descriptive variable name (sessionContext) for clarity.

var options = new SmtpServerOptionsBuilder()
    .ServerName("SMTP Server")
    .Port(9025)
    .CustomSmtpGreeting(MyCustomSmtpGreeting)
    .Build();

private static string MyCustomSmtpGreeting(ISessionContext sessionContext)
{
    return $"220 {sessionContext.ServerOptions.ServerName} hello";
}