[tex-eplain] update for CTAN lion file location

[Van Ly]

> From: Karl Berry
>
>    [1] https://www.ctan.org/lion/files/ctan_lion_350x350.png
>
> Thanks. I updated the Makefile. Seems like it would be better not to
> require that particular image, but just going with it ... --best, karl.
>

Thanks, I'm for keeping the image in the demo as is.  There is room 
to add a second image, but in color.  Something like 
https://jabcode.org/ seems useful. -v