[l2h] Broken references to {blue, green, ...}ball.svg in documentation

[Carsten Leonhardt]

Hi Dan,

> The Makefile in docs specifies "-image_type png".
> It looks like the debian build script overrides this.

indeed. One of my predecessors in packaging latex2html based the flags
on the Makefile.frames in docs/ which seems to be stale.

I've fixed it in the build script, thanks!

If the only reason the Makefile.frames exists is to add ",frames" to the
flags, I guess I can come up with something so the extra makefile can be
deleted.

Best Regards,

Carsten