Adds missing closing section tag around highlights.

There was an unbalanced `<section>` tag, opened after the highlights anchor [L696](https://github.com/google/web-starter-kit/blob/master/app/styleguide/index.html#L696). The subsequent indentation suggests that only the first `<div class="code-sample">` block should be within this `<section>`, but this PR closes the tag after the life examples and before the code examples, consistent with the other sections.

Add `w=1` to the pull request's URL to ignore the correspondingly fixed indentation and see the actual tag.