Fix SVG file for Firefox

This uses a newer version of LibreOffice to build the SVG files.

~~~
$ ls fig/*.svg
fig/file-mapping.svg  fig/forking.svg  fig/repository-links.svg
$ libreoffice --version
LibreOffice 6.0.1.1 00m0(Build:1)
$ for i in *.odg; do libreoffice --convert-to svg $i; done
~~~