doc: fix fs.openSync() signature

The `mode` argument depends on the `flags` argument but it is optional
even if the `flags` argument is specified.

PR-URL: nodejs#38591
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

doc/api/fs.md

@@ -4533,7 +4533,7 @@ and cleaning up the directory.
 The `encoding` option sets the encoding for the `path` while opening the
 directory and subsequent read operations.
 
-### `fs.openSync(path[, flags, mode])`
+### `fs.openSync(path[, flags[, mode]])`
 <!-- YAML
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