Update nuke_if_too_big documentation (Cherry-pick of #17429)

[benjyw]

• As of pants 2.13 the .pants/setup directory is approximately 300mb big, resulting in the example always clearing the directory.
• Updated the script to pass shellcheck.

Previous implementation failed with:

Line 4:
    size_mb=$(du -m -d0 ${path} | cut -f 1)
                        ^-- SC2086 (info): Double quote to prevent globbing and word splitting.

  Did you mean: (apply this, apply all SC2086)
    size_mb=$(du -m -d0 "${path}" | cut -f 1)

  Line 5:
    if (( ${size_mb} > ${limit_mb} )); then
          ^-- SC2004 (style): $/${} is unnecessary on arithmetic variables.
                       ^-- SC2004 (style): $/${} is unnecessary on arithmetic variables.

  Line 7:
      rm -rf ${path}
             ^-- SC2086 (info): Double quote to prevent globbing and word splitting.

  Did you mean: (apply this, apply all SC2086)
      rm -rf "${path}"