Fixed README.md API transaction example

[KlemenKozelj]

Hi first of all thank you for the amazing NPM.

I have noticed that In the Transaction API example, you need to pass in the session information as 3th argument:

https://www.mongodb.com/blog/post/quick-start-nodejs--mongodb--how-to-implement-transactions

It would be cool that this is included in current README.md to avoid any confusion.

Checklist

• [x] npm test passes and has 100% coverage
• [x] README.md is updated