Update fs.markdown

Inscrease informations : "mainly if you watch a file but for a folder, it is unlikely."

On mac (Darwin Kernel Version 13.2.0) : 

## Watcher on directory :

	require('fs').watch("./", function() {
		console.log(arguments);
	});

Output : ` { '0': 'change', '1': 'toto.js.lk' }`

## Watcher on file :

	require('fs').watch("./toto.js.lk", function() {
		console.log(arguments);
	});

Output : `{ '0': 'change', '1': null }`