doc: Fixes some minor styles in README.md

Fixes some minor styles in README.md

Type of change:

[ ] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] This change requires a documentation update

[x] I have signed the MongoDB CLA
[x] I have added tests that prove my fix is effective or that my feature works
[x] I have added any necessary documentation (if appropriate)
[x] I have run make fmt and formatted my code
[x] I have tested the CDK constructor in a CFN stack. See TESTING.md
[x] If changes include removal or addition of 3rd party GitHub actions, I updated our internal document. Reach out to the APIx Integration slack channel to get access to the internal document.