Vignette title errors

The vignette indices do not match the intended vignette titles, e.g.

```
title: Using the `foreach` package
author: Steve Weston
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{foreach}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{utf8}
---
```

There `title` and `%\VignetteIndexEntry{}` do no agree and the latter is what's used by R.
