Index header for files should be inserted only once

In the current template multiple @fileOverviews generates multiple "Index" headers, one for each file, rather than a single "Index" header for all files.

I'm not sure if this is intentional, however, I prefer only a single Index header.

The pull request is mainly to highlight the specific issue as I'm new to JSDoc and Docstrap and I'm unsure whether this approach makes sense as a general fix or not. It worked in my particular case so I felt it was at least worth sharing.