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.

The diff, ignoring correspondingly fixed indentation, can be seen at by appending [`w=1`](https://github.com/google/web-starter-kit/pull/152/files?w=1) to the URL.