AZW3 Output: Fix a bug that caused popup footnotes in some AZW3 files to not work properly. The popup would show all remaining footnotes instead of only the current footnote. Fixes #1293290 [popup Comments can not be displayed correctly in azw3 file](https://bugs.launchpad.net/calibre/+bug/1293290)

This was caused by the offsets to links in the AZW3 generated by calibre
to be slightly incorrect, confusing the heuristic algorithm used by the
kindle to detect footnote extents.