doc: add missing backticks to doc/api/tls.md

PR-URL: https://github.com/nodejs/node/pull/13394
Fixes: https://github.com/nodejs/node/issues/13382


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>