doc: fixed some minor styles in README.md to improve readability

[svennergr]

Proposed changes

Just some minor markdown changes to improve readability.

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

Required Checklist:

• [x] I have signed the MongoDB CLA
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] I have added any necessary documentation (if appropriate)
• [ ] I have run make fmt and formatted my code
• [ ] I have tested the CDK constructor in a CFN stack. See TESTING.md
• [ ] 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.

Further comments

[lantoli]

thanks @svennergr for your contribution. I've created a new PR so I can run all PR checks: https://github.com/mongodb/awscdk-resources-mongodbatlas/pull/298

I've cherry-picked your commit so you'll appear as a co-author.

thanks again