Change magnetURI to torrentId

Change magnetURI to torrentId to avoid confusing, because client.add is not limited only to magnet URIs and there are many ways to define torrentId, e.g. HTTP(S) URL to a *.torrent file.

docs/get-started.md

@@ -88,9 +88,9 @@ var fs = require('fs')
 
 var client = new WebTorrent()
 
-var magnetURI = 'magnet:...'
+var torrentId = 'magnet:...'
 
-client.add(magnetURI, function (torrent) {
+client.add(torrentId, function (torrent) {
   torrent.files.forEach(function (file) {
     console.log('Started saving ' + file.name)