fix(docs): update readme to use `hostname`

[soub4i]

I tried to follow the documentation to connect the SMTP client but failed because of typescript error :

error: TS2345 [ERROR]: Argument of type '{ host: string; port: number; username: string; password: string; }' is not assignable to parameter of type 'ConnectConfig'.
  Object literal may only specify known properties, and 'host' does not exist in type 'ConnectConfig'.
  host: "smtp.gmail.com",

So I updated the documentation to align with the current connectConfig

[phocks]

I had to add these too but yep good work

username?: string;
password?: string;