docs: Update step_1.md to include imports for Canvas (#3351)

I have added the import required for the Canvas component. This tripped
me up originally as I was following this example and had to go find out
what import to use for the line `void render(Canvas canvas) {`

I did not make an issue since this is an extremely small issue and is
regarding documentation and not any real code.

doc/tutorials/space_shooter/step_1.md

@@ -35,6 +35,7 @@ implemented as follows:
 
 ```dart
 import 'package:flame/components.dart';
+import 'package:flutter/material.dart';
 
 class Player extends PositionComponent {
   static final _paint = Paint()..color = Colors.white;