Fix link to LICENSE.md in README

[crweiner]

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

The link to the LICENSE file in README.md points to LICENSE.txt when the actual MIT license file is LICENSE.md.

This PR fixes it to be the proper link. 👍

[SendGridDX]

All committers have signed the CLA.

[thinkingserious]

Hello @crweiner,

Thanks again for the PR!

We appreciate your contribution and look forward to continued collaboration. Thanks!

Team SendGrid DX