doc: bump punycode api stability to 'stable'

The punycode library has been in the tree for over three years now and
has been de facto stable for all that time, if not perhaps de jure.
Let's make it official.

PR-URL: #470
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>

doc/api/punycode.markdown

@@ -1,6 +1,6 @@
 # punycode
 
-    Stability: 2 - Unstable
+    Stability: 3 - Stable
 
 [Punycode.js](https://mths.be/punycode) is bundled with io.js v1.0.0+ and
 Node.js v0.6.2+. Use `require('punycode')` to access it. (To use it with