docs: fix broken image links on npm

[IObert]

Fixes

Closes #1343

There's a broken link on the npm pages of each package because npm isn't aware of the notion of the monorepo. Hard-coding the URL to the image is the easiest solution to this problem.

https://www.npmjs.com/package/@sendgrid/client

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the main branch
• I have added tests that prove my fix is effective or that my feature works -> Not applicable
• I have added the necessary documentation about the functionality in the appropriate .md file -> Not applicable
• I have added inline documentation to the code I modified -> Not applicable

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