yes, the (+) and (-) connections are correctly marked. Connect the power lead from the ignition switch to the (+), the wire from the circuit breaker to the (-).
Another "but"; it would not take much time or effort to try it both ways and see if it makes a difference in the appearance of the spark for YOUR equipment. There is a big difference in the appearance and behavior of spark jumping the gap in open air as opposed to in the cylinder under compression, but it is REALLY hard to see under actual operating conditions

, so the only visual comparison you can do is with the plugs out. Try it, tell us what you see.
The bottom line is that it will work either way, and won't hurt the coil or any of the other stuff, but it might make a little difference in performance if the plug polarity is working in the direction that the MG Club article suggests.
good luck!, hope she runs great!
mike