doc: add parentheses to refreshTmpDir()

`common.refreshTmpDir()` is a function. Add parentheses to make that
explicit. Also copy edit the descriptive sentence.

PR-URL: https://github.com/nodejs/node/pull/16168


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>