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: #38591
Reviewed-By: Antoine du Hamel <[email protected]>
Reviewed-By: Michaël Zasso <[email protected]>
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Darshan Sen <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: Trivikram Kamat <[email protected]>
Reviewed-By: Zijian Liu <[email protected]>
Reviewed-By: Benjamin Gruenbaum <[email protected]>

doc/api/fs.md

@@ -4501,7 +4501,7 @@ 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]])`
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