bug#41418: Rendered graphs in manual have improperly rendered text

[Carlo Zancanaro]

I was looking at the manual for the guix graph command today, and 
I noticed that the text in the images isn't being rendered 
properly. Instead of legible characters, it's just boxes.

For example: 
https://guix.gnu.org/manual/en/html_node/Invoking-guix-graph.html#Invoking-guix-graph

I looked through all the images and it looks like only the graphs 
are affected (ie. the installer screenshots and guix size output 
looked fine), however all of the graphs are affected.