Use minimal example json to show how to import .json files

It's worth doing but currently I get the following error message:

```
cargo run --bin import_traffic -- \
--map=data/system/us/seattle/maps/montlake.bin \
--input=minimal_scenario2.json
Finished dev [unoptimized + debuginfo] target(s) in 0.13s
Running `target/debug/import_traffic --map=data/system/us/seattle/maps/montlake.bin --input=minimal_scenario2.json`
import traffic demand data...
Read data/system/us/seattle/maps/montlake.bin (3)... 0.4303s
parse minimal_scenario2.json...
parse minimal_scenario2.json took 0.0001s
thread 'main' panicked at 'Couldn't read_json(minimal_scenario2.json): invalid type: floating point `10800`, expected i32 at line 7 column 30', /mnt/57982e2a-2874-4246-a6fe-115
```