Update CONTRIBUTING.md

Remove some redundant words.

CONTRIBUTING.md

@@ -12,15 +12,15 @@ Things you will need
  * An ssh client (used to authenticate with GitHub).
  * Python (used by some of our tools).
  * The Dart SDK (see [Issue #54](https://github.com/flutter/flutter/issues/54)
-   about downloading the Dart SDK automatically). Note: If you're also
+   about downloading the Dart SDK automatically). _If you're also
    working on the Flutter engine, you can use the copy of the Dart SDK
-   in `.../engine/src/third_party/dart-sdk/dart-sdk`.
+   in `.../engine/src/third_party/dart-sdk/dart-sdk`._
    - [https://www.dartlang.org/downloads/archive/](https://www.dartlang.org/downloads/archive/)
  * The Android platform tools (see [Issue #55](https://github.com/flutter/flutter/issues/55)
    about downloading the Android platform tools automatically).
-   Note: If you're also working on the Flutter engine, you can use the
+   _If you're also working on the Flutter engine, you can use the
    copy of the Android platform tools in
-   `.../engine/src/third_party/android_tools/sdk/platform-tools`.
+   `.../engine/src/third_party/android_tools/sdk/platform-tools`._
    - Mac: `brew install android-platform-tools`
    - Linux: `sudo apt-get install android-tools-adb`