docs: Fixes documented example code

[tom-spalding]

Fixes

Hello, this resolves the issue described in https://github.com/sendgrid/sendgrid-ruby/issues/438. It seems like some of the example code provided by the Sendgrid's integration guide needs to be modified a bit. The same code is found in the examples of this PR.

Thanks for the great product!

Checklist

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