Set terrain arg to store true.

[benjamin051000]

This was erroneously set to store false, which means it's impossible to enable the terrain from the cli.

[benjamin051000]

Hm, this is failing with the following:

cargo r --no-default-features --release -- --bbox "-71.035194,42.327427,-71.008286,42.342210" --path "/home/benjamin/.var/app/org.prismlauncher.PrismLauncher/data/PrismLauncher/instances/Fabulously Optimized(3)/.minecraft/saves/testing" --terrain

(with a dbg! around the first line of ground::fetch_elevation_data) Prints:

[src/ground.rs:130:32] bbox_str.split_whitespace().map(|s: &str|
s.parse::<f64>()).collect::<Result<Vec<f64>, _>>() = Err(
    ParseFloatError {
        kind: Invalid,
    },
)
Warning: Failed to fetch elevation data: invalid float literal

[benjamin051000]

I'll add a unit test that verifies this before merging.

[benjamin051000]

Blocked on #378

[benjamin051000]

@louis-e once CI passes, this is ready for signoff

[louis-e]

So I guess we're starting with unit tests now ;)
Thanks a lot :)