Update redme, fix code example

[nilsmagnus]

📢 Type of change

• Bugfix
• New feature
• [ x] Enhancement
• Refactoring

📜 Description

Error in code example in readme

💡 Motivation and Context

The example in the readme was missing an async keyword to compile.

📝 Checklist

• [ x] I reviewed submitted code
• I added tests to verify changes
• All tests passing
• [x ] No breaking changes

🔮 Next steps

[marandaneto]

thanks for this :)

try out our alpha release btw:
so you don't have to do this manually:

https://github.com/getsentry/sentry-dart/releases/tag/4.0.0-alpha.2