Replace :root by html in plugin docs

This is a mandatory change for Polymer 2 compatibility:
https://polymer-library.polymer-project.org/2.0/docs/upgrade#wrap-custom-style-elements

Change-Id: I17721edb7b31a75bb165c5a3a1631b6287ce9f3e

Documentation/pg-plugin-styling.txt

@@ -25,7 +25,7 @@ Plugins should provide Style Module, for example:
   <dom-module id="some-style">
     <template>
       <style>
-        :root {
+        html {
           --css-mixin-name: {
             property: value;
           }