Use KeyboardType.phone for the Gallery app's text demo. (#3983)

BUG=https://github.com/flutter/flutter/issues/3970

examples/flutter_gallery/lib/demo/text_field_demo.dart

@@ -82,6 +82,7 @@ class TextFieldDemoState extends State<TextFieldDemo> {
             new Input(
               hintText: 'Where can we reach you?',
               labelText: 'Phone Number',
+              keyboardType: KeyboardType.phone,
               formField: new FormField<String>(
                 setter: (String val) { person.phoneNumber = val; },
                 validator: _validatePhoneNumber