docs: fix usage link in README

[BogdanHabic]

Fixes

This PR fixes a typo that was present in the README. The use cases link wasn't correct.

Checklist

• I confirm that the latest PR commit passes its Travis CI build.
• 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:

• Updated usage link in readme

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[SendGridDX]

All committers have signed the CLA.

[thinkingserious]

Hello @BogdanHabic,

Thanks again for the PR!

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Team SendGrid DX

[BogdanHabic]

Hey @thinkingserious, sure thing! Great DX initiative 😄