doc: 'constructor' implies use of new keyword

The square module is described as exporting a constructor,
which would mean it would need to be invoked with the
new keyword in bar.js after requiring it. Otherwise it's
technically a factory function, not a constructor.

PR-URL: https://github.com/nodejs/node/pull/17364


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>