Missing title in the generated HTML

[rffontenelle]

docs/tools/update_html.py displays the follow warning for each language, and the web broswer's tab shows "index" as page title.

[WARNING] This document format requires a nonempty <title> element.
  Defaulting to 'index' as the title.
  To specify a title, use 'title' in metadata or --metadata title="...".