doc: improve punctuation in fs.open() text

PR-URL: #17463
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

doc/api/fs.md

@@ -1261,7 +1261,7 @@ The file is created if it does not exist.
 * `'ax+'` - Like `'a+'` but fails if `path` exists.
 
 `mode` sets the file mode (permission and sticky bits), but only if the file was
-created. It defaults to `0666`, readable and writable.
+created. It defaults to `0o666` (readable and writable).
 
 The callback gets two arguments `(err, fd)`.