Update styletron example (#5879)

Missed a fix for the styletron api. Previous config won't work for server rendering.

examples/with-styletron/pages/_document.js

@@ -16,8 +16,8 @@ export default class MyDocument extends Document {
           {this.props.stylesheets.map((sheet, i) => (
             <style
               className='_styletron_hydrate_'
-              dangerouslySetInnerHTML={{ __html: sheet.css }}
-              media={sheet.media || null}
+              dangerouslySetInnerHTML={{ __html: sheet }}
+              media={sheet.attrs ? sheet.attrs.media : null}
               key={i}
             />
           ))}