Fix docstring variable name

[delirious-lettuce]

Fixes

Checklist

• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the [Contribution Guide] and my PR follows them.
• [x] 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:

This PR changes the docstring variable name from response -> request_headers to match the actual name of the method parameter.

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

[thinkingserious]

Thanks @delirious-lettuce! I've added this to our backlog for review.

[thinkingserious]

Hello @delirious-lettuce,

Thanks again for the PR!

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

Team SendGrid DX