Add clarification about micro:bit REPL

[carlosperate]

Taking in consideration the micro:bit could have a MakeCode or Mbed program on the nRF51 it's quite easy to open the REPL and not see anything appear.
We could add a small clarification to https://codewith.mu/en/tutorials/1.0/repl to indicate that MicroPython needs to be flashed to the micro:bit for the REPL to work.