One-tile option doesn't produce tiles above zoom level 20

Hi,
I noticed that when I use tippecanoe to produce just a single tile above zoom level 20, it fails with an error:
example usage:
cmd: `tippecanoe -R 24/257/286 -o mvtServer/24_257_286.mvt sample_geojson.json`
error: `Minimum zoom -Z24 cannot be greater than maxzoom -z20`

works fine under zoom level 20

Start by reproducing the reported command, `tippecanoe -R 24/257/286 -o mvtServer/24_257_286.mvt sample_geojson.json`, and trace how the `-R` option sets the zoom range. Check the handling of requests above zoom 20 and verify that a one-tile request such as this completes without the minimum-zoom error.

Written by the indexing model from the issue text.