use rem instead of em for examples

Not safe to use em since if you don't have any explicit font-size defined, it just do nothing.

Here was a example where using em didn't work

```html
<style>
@import "../node_modules/cssrecipes-custom-media-queries/index.css";
@import "../node_modules/cssrecipes-grid/index.css";
@import "../node_modules/cssrecipes-utils/lib/all.css";
@import "../node_modules/cssrecipes-utils/lib/max.css";

.r-Grid {
  width: 50em;
}

</style>
<body>
  <div class="r-Grid">
    <h1 class="r-Grid-cell">
      Hi there
    </h1>
  </div>
```

.r-Grid have width == 0 :( (confirmed on firefox & chrome)