inconsistencies in population size showing in the latest tutorial

[biozzq]

Dear @stschiff

The factor 2 will cause the inconsistencies when plotting effective population size. And this has been disscussed here https://github.com/stschiff/msmc2/issues/17#issuecomment-433145181. You should also update the Ne plot in the latest tutorial. Thank you very much.

lines(afrDat$left_time_boundary/mu*gen, (1/afrDat$lambda)/(2*mu), type="s", col="red")

Sincerely, Zheng zhuqing