doc: updated fs.ftruncate() docs to reflect mandatory callback parameter

Author: Mert Can Altin

doc/api/fs.md

@@ -3029,7 +3029,7 @@ changes:
 
 * `fd` {integer}
 * `len` {integer} **Default:** `0`
-* `callback` {Function}
+* `callback` {Function} (**Required**)
   * `err` {Error}
 
 Truncates the file descriptor. No arguments other than a possible exception are