doc,fs: keep consistent with fs.link and fs.linkSync

[yorkie]

So firstly, the destination has a little confusion like I got the log:

Error: EEXIST: file already exists, symlink 'dist/node_modules' -> 'node_modules'

And my source call is:

fs.symlinkSync('dist/node_modules', 'node_modules', 'dir');

Because I thought the first argument stands for destination as the original doc is telling, but it actually is source. Plus, I also make link* and symlinkSync be consistent :-)