Use class selector instead of element selectors

As far as I can tell `css-modules` locally scope class selectors but not
element selectors in from the component css file. This means that the
`GlobalSelectors` example is slightly misleading since:

```
.root :global p {
  ...
}
```

and

```
.root p {
  ...
}
```

Both have the desired result of styling all the `p` tags inside the
elements with the `.root` class applied.

Updating the example to use a, semantically meaningless, class makes
things clearer for people going through the examples.

Author: lengarvey

src/components/2-GlobalSelectors/GlobalSelectors.css

@@ -7,7 +7,7 @@
   max-width: 400px;
 }
 
-.root :global p {
+.root :global .text {
   color: brown;
   font-size: 24px;
   font-family: helvetica, arial, sans-serif;

src/components/2-GlobalSelectors/GlobalSelectors.js

@@ -7,7 +7,7 @@ export default class GlobalSelectors extends Component {
   render() {
     return (
       <div className={ styles.root }>
-        <p>Global Selectors</p>
+        <p className="text">Global Selectors</p>
       </div>
     );
   }