readme: new url to polymer-modulizer

[milahu]

polymer/README.md

Line 122 in 1e8b246

However, the [`polymer-modulizer`](https://github.com/Polymer/polymer-modulizer) tool automates the vast majority of this migration work. Please see details on that repo for automated conversion of Polymer 2.0 apps and elements to Polymer 3.0.

new url

https://github.com/Polymer/tools/tree/master/packages/modulizer