update README.md, generate .svg file is better

[ttys3]

dot generate svg file for the project just took 0.269s,
but took 5.286s for png file
and also , svg is better for resizing the image.

the display command from imagemagick package is not good at displaying huge image,
so it's better to use xdg-open let the Desktop environment to choose

❯ time go mod graph | modv | dot -T svg | display
go mod graph  0.06s user 0.03s system 186% cpu 0.048 total
modv  0.01s user 0.00s system 10% cpu 0.051 total
dot -T svg  0.22s user 0.01s system 83% cpu 0.269 total
display  46.03s user 4.21s system 215% cpu 23.267 total

❯ time go mod graph | modv | dot -T png | display
go mod graph  0.07s user 0.02s system 178% cpu 0.051 total
modv  0.00s user 0.00s system 10% cpu 0.054 total
dot -T png  5.12s user 0.11s system 99% cpu 5.276 total
display  39.50s user 3.75s system 156% cpu 27.591 total