Update doc vars

[lee1043]

Describe your documentation update

In this example for the spatial average, variable tas is used for deriving Nino 3.4 index. However in CMIP, the variable name tas has been indicating near surface (2-m) temperature, I think it would make more sense to use ts variable instead, which is for surface temperature (thus considered as sea-surface temperature over ocean).

I suggest replacing tas by ts.

[tomvothecoder]

Another doc caught in xCDAT/xcdat-validation#27 to be done with this issue:
data_var should replace keep_var