Update declaration

Author: Masth0

package.json

@@ -1,6 +1,6 @@
 {
   "name": "scratchcard-js",
-  "version": "1.5.3",
+  "version": "1.5.4",
   "description": "Create scratch card in browser.",
   "main": "./build/scratchcard.min.js",
   "scripts": {

types/ScratchCard.d.ts

@@ -1,11 +1,12 @@
 import { SC_CONFIG, SCRATCH_TYPE } from './ScratchCardConfig';
 declare class ScratchCard {
+    get canvas(): HTMLCanvasElement;
     readonly config: SC_CONFIG;
     readonly scratchType: SCRATCH_TYPE;
     readonly ctx: CanvasRenderingContext2D;
     readonly container: HTMLElement;
     private position;
-    private canvas;
+    private _canvas;
     private readyToClear;
     private brush;
     private callbackDone;