doc: small improvements in readline code examples

1. Consistent template literals in `console.log()`.
2. === instead of ==.
3. const instead of var.

PR-URL: https://github.com/nodejs/node/pull/9628


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Jackson Tian <shyvo1987@gmail.com>