docs: update the link of the Repl.it badge
docs: update the link of the Repl.it badge

The link will now point towards a sample project, instead of the root
repository.

Related: https://github.com/socketio/socket.io/issues/3934

Automattic/socket.io,mosoft521/socket.io,mosoft521/socket.io,socketio/socket.io,Automattic/socket.io,socketio/socket.io