[docs] Clarify private messaging in the emit cheatsheet (#3232)

The previous version was confusing, as `socket.to(socket.id).emit()` does nothing.

Fixes #3220

docs/emit.md

@@ -29,7 +29,7 @@ function onConnect(socket){
   io.of('myNamespace').to('room').emit('event', 'message');
 
   // sending to individual socketid (private message)
-  socket.to(<socketid>).emit('hey', 'I just met you');
+  io.to(<socketid>).emit('hey', 'I just met you');
 
   // sending with acknowledgement
   socket.emit('question', 'do you think so?', function (answer) {});