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: nodejs#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>

doc/api/readline.markdown

@@ -69,7 +69,7 @@ nothing is displayed.
 Example usage:
 
 ```js
-interface.question('What is your favorite food?', (answer) => {
+rl.question('What is your favorite food?', (answer) => {
   console.log(`Oh, so your favorite food is ${answer}`);
 });
 ```