doc: don't use "interface" as a variable name

In readline.markdown, don't use strict mode reserved keyword "interface"
as a variable name.

This commit changes the name of one `readline.Interface` instance from
"interface" to "rl", as it is named in other places of the doc.

PR-URL: https://github.com/nodejs/node/pull/4900


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>