lib: correct typo in createSecureContext

Minor correction in the comment regarding ssl_set_pkey.

PR-URL: https://github.com/nodejs/node/pull/13653


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>