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>

doc/onboarding.md

@@ -32,7 +32,7 @@ onboarding session.
 ## Local setup
 
   * git:
-    * make sure you have whitespace=fix: `git config --global --add core.whitespace fix`
+    * make sure you have whitespace=fix: `git config --global --add apply.whitespace fix`
     * usually PR from your own github fork
     * [See "Updating Node.js from Upstream"](./onboarding-extras.md#updating-nodejs-from-upstream)
     * make new branches for all commits you make!