typo with pixel density

Author: lauren mccarthy

lib/p5.js

@@ -89,7 +89,7 @@ var core = function (require, shim, constants) {
     var constants = constants;
     var p5 = function (sketch, node) {
       this._setupDone = false;
-      this._pixel = window.devicePixelRatio || 1;
+      this._pixelDensity = window.devicePixelRatio || 1;
       this._startTime = new Date().getTime();
       this._userNode = node;
       this._curElement = null;