Update to Stage 3

During [T39 2017 July meeting](https://github.com/tc39/agendas/blob/master/2017/07.md) the spec was approved to be Stage 3.

Author: exarus

spec.html

@@ -2,7 +2,7 @@
 <meta charset="utf8">
 <pre class=metadata>
 title: Integrated public and private fields proposal
-stage: 2
+stage: 3
 contributors: Daniel Ehrenberg, Jeff Morrison, Kevin Smith, Kevin Gibbons
 </pre>
 <style>