Fix start.tessel.io view of example file

On the start.tessel.io page (or more specifically, this page: http://start.tessel.io/modules/rfid) you can still see the text `// Replace '../' with 'rfid-pn532' in your own code`, even though the code for the start.tessel.io site tries to strip this away. The reason is that this module is lacking a space in the comment.

This PR fixes that, and I have tested it against the start.tessel.io codebase and verified the page in question will render as expected.