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>

test/common/README.md

@@ -317,10 +317,10 @@ Port tests are running on.
 
 Logs '1..0 # Skipped: ' + `msg`
 
-### refreshTmpDir
+### refreshTmpDir()
 * return [<String>]
 
-Deletes the 'tmp' dir and recreates it
+Deletes the testing 'tmp' directory and recreates it.
 
 ### restoreStderr()