fs: fix the error report on fs.link(Sync)

As the Node.js documentation specified:

> fs.link(srcpath, dstpath, callback)

But when we run:

```js
> fs.link()
```

We will get the below:

```js
TypeError: dest path must be a string
    at TypeError (native)
    at Object.fs.link (fs.js:915:11)
    at repl:1:4
```

Just correct the message of relevant 2 errors in this PR :-)

PR-URL: https://github.com/nodejs/node/pull/3917


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>