Renamed doc to docs and minor language fixes

docs/examples/nine_tile_box/lib/main.dart

@@ -0,0 +1,35 @@
+import 'package:flame/game.dart';
+import 'package:flame/flame.dart';
+import 'package:flame/nine_tile_box.dart';
+import 'package:flame/sprite.dart';
+
+import 'package:flutter/material.dart';
+
+void main() async {
+  WidgetsFlutterBinding.ensureInitialized();
+  final size = await Flame.util.initialDimensions();
+
+  final game = MyGame(size);
+  runApp(game.widget);
+}
+
+class MyGame extends Game {
+  Size size;
+  NineTileBox nineTileBox;
+
+  MyGame(this.size) {
+    final sprite = Sprite('nine-box.png');
+    nineTileBox = NineTileBox(sprite, tileSize: 8, destTileSize: 24);
+  }
+
+  @override
+  void render(Canvas canvas) {
+    const length = 300.0;
+    final x = (size.width - length) / 2;
+    final y = (size.height - length) / 2;
+    nineTileBox.draw(canvas, x, y, length, length);
+  }
+
+  @override
+  void update(double t) {}
+}