RFM2Pi Gateway: fix readme -> read logs

- Typo in path
- No need to sudo
- Refresh option is capital F, as the file is rotated (otherwise, at rotation time, the refresh stops)
- (The number of lines is just the one I'm used to. I copy-pasted the correct line from my usual command line. I guess anyone can figure that out and change it anyway.)

I chose a rotation on two log files to allow for a GUI frame to always display the maximum number of lines. With only one file, there would be a point in time where only 1 line would be available. Considering the file size, this would not happen that often, so I'm not sure we care, but anyway, the code is here and anyone can parameter the number and size of the log files.