doc: correct git fix whitespace command

Use apply.whitespace=fix rather than core.whitespace=fix

Ref: https://github.com/nodejs/node/issues/11412
PR-URL: https://github.com/nodejs/node/pull/12445


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>