[chore] fix incorrect path for module in package.json.

Author: diasbruno

Diff for: package.json

@@ -3,7 +3,7 @@
   "version": "3.4.2",
   "description": "Accessible modal dialog component for React.JS",
   "main": "./lib/index.js",
-  "module": "./src/index.js",
+  "module": "./lib/index.js",
   "repository": {
     "type": "git",
     "url": "https://github.com/reactjs/react-modal.git"