scripts/docs-spellcheck.sh uses set -u and constructs args=( ... "${write_flag[@]}" ), where write_flag may be an empty array. On bash 3.2 (still the default /bin/bash on macOS), referencing an empty array under set -u raises an unbound variable error. Newer bash (>= 4.4) handles this expression correctly, which is why the script ships green on Linux CI runners.
Switch to the bash 3.2-safe parameter expansion ${write_flag[@]+"${write_flag[@]}"}: it expands to nothing when the array is empty and to the array contents otherwise, preserving --write behavior unchanged.
Also fixes overrideable -> overridable in docs/reference/test.md, which the now-running spellcheck surfaces.
Repro:
bash scripts/docs-spellcheck.sh # was: write_flag[@]: unbound variable, exit 1
bash scripts/docs-spellcheck.sh # now: codespell runs to completion
