Minor grammar fixes. (#9808)

* Minor grammar fixes.

* Update README.md

packages/flutter/README.md

@@ -2,7 +2,7 @@ Flutter
 =======
 
 Flutter is a new way to build high-performance, cross-platform mobile apps.
-Flutter is optimized for today's, and tomorrow's, mobile devices. We are
+Flutter is optimized for today's — and tomorrow's — mobile devices. We are
 focused on low-latency input and high frame rates on Android and iOS.
 
 See the [getting started guide](https://flutter.io/getting-started/) for