docs: Fixed a broken link to the api reference and added an ES8 syntax code example.

[camerondaly]

Fixes

Hi! This is my first time contributing here -- I apologize if I've gone about any part of this incorrectly. I'm happy to follow up and resolve any issues.

I'm proposing one fix and one addition:

1. The link on line 49 in the README points to a page that doesn't exist (404). I've provided a link to what I think is the new home for the API reference.

• Old link
• New link

2. From what I've seen, the docs/README's here often provide both ES6 and ES8 examples, so I went ahead and added an ES8 snippet to correspond with the existing ES6 snippet.

Hopefully this is helpful! I'm enjoying using SendGrid in my own projects. Thanks!

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the main branch
• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

If you have questions, please file a support ticket, or create a GitHub Issue in this repository.