src: correct endif comment SRC_NODE_API_H__

Really minor but I could not find an open PR for anything n-api where
this could be changed, so creating this so that it is not forgotten.

PR-URL: https://github.com/nodejs/node/pull/13190


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <mhdawson@ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>