update js-apis-fileio.md

Signed-off-by: Nraoxian <raoxian@huawei.com>

zh-cn/application-dev/reference/apis/js-apis-fileio.md

@@ -603,9 +603,9 @@ openSync(path:string, flags?:number, mode?:number): number
   ```js
   let fd = fileio.openSync(path, 0o102, 0o666);
   fileio.writeSync(fd, 'hello world');
-  let fd = fileio.openSync(path, 0o2002);
-  fileio.writeSync(fd, 'hello world');
-  let num = fileio.readSync(fd, new ArrayBuffer(4096), {position: 0});
+  let fd1 = fileio.openSync(path, 0o2002);
+  fileio.writeSync(fd1, 'hello world');
+  let num = fileio.readSync(fd1, new ArrayBuffer(4096), {position: 0});
   console.info("num == " + num);
   ```