Table dash count subtraction with overflow error

[dsherret]

https://github.com/littletof/prettyBenching/blob/696e3541fc5a52e35776f93c704ddae31a1a3d44/README.md

thread 'test_specs' panicked at 'attempt to subtract with overflow', src\parsing\parse.rs:638:32

Problem line:

let dashes_count = column_width - (if has_left_colon { 1 } else { 0 }) - (if has_right_colon { 1 } else { 0 });