doc: change ./node to node in debugger.md

Fixes: https://github.com/nodejs/node/issues/8942
PR-URL: https://github.com/nodejs/node/pull/8943


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>