Change dev to beta (flutter#837)

Fixes flutter/flutter#14930

upgrading.md

@@ -4,14 +4,14 @@ title: Upgrading Flutter
 permalink: /upgrading/
 ---
 
-We strongly recommend tracking the `dev` branch in the flutter repository, which
+We strongly recommend tracking the `beta` branch in the flutter repository, which
 is where we push 'known good builds' of Flutter. If you need to 
 view the very latest changes, you can track the `master` branch, but note this is where
 we do our daily development, so stability is much lower.
 
 To view your current branch, use `flutter channel`.
 
-To change branch, use `flutter channel dev` / `flutter channel master`.
+To change branch, use `flutter channel beta` / `flutter channel master`.
 
 ## Specifying the Flutter SDK for your project