Fix broken 2D array reference

I (somehow) forgot how to properly reference a "cell" in a 2D array...

starfish.js

@@ -194,7 +194,7 @@ class CodeBox {
     }
 
     Swim() {
-        const instruction = this.box[this.pointer[0], this.pointer[1]];
+        const instruction = this.box[this.pointer[0]][this.pointer[1]];
 
         if(this.stringMode != 0 && instruction != this.stringMode) {
             this.stack.Push(dec(instruction));