Typo: sitemap__index_header

```
See: https://bugs.launchpad.net/bugs/670186

There's a typo, and one branch of an if-else sets sitemap__index_header instead 
of sitemap_index_header.
```

Original issue reported on code.google.com by `stefano.rivera` on 6 Nov 2010 at 3:18

Attachments:
- [sitemapgen.diff](https://storage.googleapis.com/google-code-attachments/sitemap-generators/issue-36/comment-0/sitemapgen.diff)
