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/api.md

@@ -298,9 +298,9 @@ Here is a usage example:
 
 ```js
 var client = new WebTorrent()
-var magnetURI = 'magnet: ...'
+var torrentId = 'magnet: ...'
 
-client.add(magnetURI, function (torrent) {
+client.add(torrentId, function (torrent) {
   // create HTTP server for this torrent
   var server = torrent.createServer()
   server.listen(port) // start the server listening to a port