Update SendGrid example construction syntax

SendGrid updated the syntax for a constructing a "sender".

```javascript
var SendGrid = require('sendgrid').SendGrid;
var sender = SendGrid(user, pass);

// Changed to

var sender = require('sendgrid')(user, pass);
```

Associated README update from sendgrid-nodejs project:
https://github.com/sendgrid/sendgrid-nodejs/commit/d5f2cece523d6c9e5a2c79ae30971cbebbb8c9c5#diff-04c6e90faac2675aa89e2176d2eec7d8