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.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc