Fantstic guide on getting all working, stuck on getting senddata to start on reboot

This is the best guide I have used, I work in schools, environmental studying, and this all just works!
I am stuck  on getting the senddata to start on reboot?

When I type sudo 

nano /etc/rc.local
python3 /home/pi/senddata.py

It opens a blank file, what do I do next.

Thanks
Tim