Fix typo in 01_rot13_server_select.c

Erroneously resetting n_written and buffer_used back to 1 would
leave a gap at the beginning of the buffer, filled with the very
first character that was received.

examples_01/01_rot13_server_select.c

@@ -110,7 +110,7 @@ do_write(int fd, struct fd_state *state)
  }
 
  if (state->n_written == state->buffer_used)
- state->n_written = state->write_upto = state->buffer_used = 1;
+ state->n_written = state->write_upto = state->buffer_used = 0;
 
  state->writing = 0;