Correct drift rate calculation in find_doppler.py

There was an off-by-one error when calculating the resolution of the drift rate. You don't want to divide by number of timesteps. Instead, you want to divide by (number of timesteps - 1). Think of the line as being between the centroid of a bin in the first and last row, rather than the very start of the first row and the end of the last row.

@lacker discussed this in the iseti meeting of 5/4/2022 and also with @stevecroft on the previous day. There is a general astronomer-consensus that this fix is an improvement.