Update null safety sample with new dev channel build (#574)

experimental/null_safety/README.md

@@ -36,7 +36,7 @@ flag](https://dart.dev/tools/experiment-flags).
 This sample is
 [experimental](https://github.com/flutter/samples/tree/master/experimental), and
 requires using a **dev channel** Flutter SDK. You can change to the dev channel
-with `flutte channel dev` followed by `flutter upgrade` (don't forget to change
+with `flutter channel dev` followed by `flutter upgrade` (don't forget to change
 back to `flutter channel stable` for production use.
 
 ### Running from the terminal/command-prompt

experimental/null_safety/null_safe_app/pubspec.lock

@@ -150,5 +150,5 @@ packages:
     source: hosted
     version: "2.1.0-nullsafety.3"
 sdks:
-  dart: ">=2.11.0-234.0 <=2.11.0-234.0.dev"
+  dart: ">=2.11.0-234.0 <=2.11.0-242.0.dev"
-  flutter: ">=1.24.0-2.0.pre.86"
+  flutter: ">=1.24.0-3.0"

experimental/null_safety/null_safe_app/pubspec.yaml

@@ -5,7 +5,7 @@ version: 1.0.0+1
 
 environment:
   sdk: ">=2.11.0-234.0 <2.12.0"
-  flutter: ">=1.24.0-2.0.pre.86"
+  flutter: ">=1.24.0-3.0"
 
 dependencies:
   flutter: