docs: Fix number of free emails in README

[mjjs]

Somewhere along the line, the number seems to have been replaced with a version number.

Fixes

Changes the README to include the number of free emails for the first 30 days instead of showing a version number there. The number has been confirmed from https://sendgrid.com/resource/faq/.

Checklist

• [x] 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)
• [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
• [ ] I have added the necessary documentation about the functionality in the appropriate .md file
• [ ] I have added inline documentation to the code I modified

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