Cleanup README.md

[domesticmouse]

This will land up after all the codelab deprecations have landed. I will need to mirror the deletions in this as well. I'm doing this mainly to reduce the number of review round trips.

Pre-launch Checklist

• I read the Effective Dart: Style recently, and have followed its advice.
• I signed the CLA.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel channel on Discord.

[gemini-code-assist]

Code Review

This pull request cleans up the README.md file by removing a trailing whitespace and un-indenting a list of codelabs for better formatting. The changes improve readability and consistency. I've found a minor inconsistency in the list where a couple of items are missing the word 'is', and I've suggested a fix to make the list uniform.