Off-by-one error in read_g2o

[keevindoherty]

The error lies here: https://github.com/MarineRoboticsGroup/mac/blob/main/examples/pose_graph_utils.py#L240-L241

The submatrix used to compute tau should be meas_info[0:3, 0:3] and for kappa should be meas_info[3:6, 3:6]. Fortunately, this doesn't seem to impact results on the benchmark datasets we've tested on before.