Fix broken bug graph image on bugstats.php

Fixes: [issue 13012](https://issues.dlang.org/show_bug.cgi?id=13012)

The original image link appears to point to a non-existent file; furthermore, the link appears to be mangled with extraneous `%3A` characters. Fixing the link led to a working graph image, so replaced the image link with the address of that image. Not 100% sure this image URL will always work, though. Maybe the problem is that the image needs to be periodically updated?