doc: incorrect backtick usage

* **Version**: master, v8.x
* **Platform**: n/a
* **Subsystem**: doc

https://nodejs.org/docs/latest/api/tls.html#tls_tls_connect_options_callback and https://nodejs.org/docs/v8.0.0/api/tls.html#tls_tls_createserver_options_secureconnectionlistener have some formatting issues in their descriptions for `NPNProtocols`. It looks like some incorrectly placed backticks.