docs(guide/scope): ensure demo CSS styles work in JSFiddle and Plunker

The CSS styling in the ng-scope demo was using CSS classes (`.doc-example-live` and
`.show-scope') to prevent the styling for the demo from affecting the entire page.
Unfortunately elements containing these classes did not appear in JSFiddle or Plunker
when you click edit.

This fix moves the `.show-scope' class inside the demo (renaming it `.show-scope-demo`)
and removes the reliance on `.doc-example-live` altogether.

Closes #4838