Correct link for environment variables setup

[ghost]

Fixes #788

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:

• Update the link for environment variables to the correct one

If you have questions, please send an email to Sendgrid, or file a Github Issue in this repository.

[SendGridDX]

All committers have signed the CLA.

[reedsa]

Thanks for the contribution! Just a quick recommendation and then I'll be happy to approve.

[reedsa]

I think we should use a relative link here instead so that GH always transforms the link properly regardless of the branch someone may be on.

[ghost]

Thank you for the feedback. Made the change to use a relative link.

[reedsa]

Perfect, thanks for the update!