fix deprecated usage of constructor with Buffer

lib/index.js

@@ -90,7 +90,7 @@ app.post('/download', async (req, res, next) => {
     res
       .contentType('text/calendar')
       .status(200)
-      .send(new Buffer(ics, 'utf8'))
+      .send(Buffer.from(ics, 'utf8'))
   } catch (err) {
     next(err)
   }