subtract overflow in ascii mode

[skyzh]

thread 'tokio-runtime-worker' panicked at 'attempt to subtract with overflow', /Users/skyzh/.cargo/registry/src/github.com-1ecc6299db9ec823/pretty-xmlish-0.1.7/src/lib.rs:253

if this line is changed to .ascii

https://github.com/risinglightdb/risinglight/blob/2143a7c93d262eb7334298e357a358b0f675bff6/src/executor_v2/explain.rs#L28

and run make apply_planner_test on macOS