small fix: empty range error in circle_packing

[joaoag]

Motivation for changes

• I was playing with the circle_packing example to better understand the egui library
• On setting both min radius and max radius to 0, the program panics and exits with cannot sample empty range
• I really value Nannou as a library, so wanted to give back a little by including a fix for this

Summary of changes

• Set minimum value of max radius to 0.1 to avoid empty range errors

PR checklist

As per the docs I have:

• run cargo fmt --all
• Errors, none shown
• Warnings, no new added
• Check Examples, have run all examples with cargo run --bin run_all_examples