[GroupedSFrame] Fixed a bug in group-info.
[GroupedSFrame] Fixed a bug in group-info.

The bug was also present in Line 183 of the following file.
(https://github.com/dato-code/SFrame/pull/183/files)

The bug did not trigger test failures because this is not a feature
exposed to the Python side.

turi-code/SFrame,turi-code/SFrame,haijieg/SFrame,TobyRoseman/SFrame,hoytak/SFrame,haijieg/SFrame,hoytak/SFrame,TobyRoseman/SFrame,haijieg/SFrame,dato-code/SFrame,turi-code/SFrame,haijieg/SFrame,hoytak/SFrame,dato-code/SFrame,TobyRoseman/SFrame,hoytak/SFrame,dato-code/SFrame,dato-code/SFrame,TobyRoseman/SFrame,haijieg/SFrame,dato-code/SFrame,haijieg/SFrame,ylow/SFrame,ylow/SFrame,dato-code/SFrame,TobyRoseman/SFrame,turi-code/SFrame,turi-code/SFrame,ylow/SFrame,dato-code/SFrame,ylow/SFrame,ylow/SFrame,TobyRoseman/SFrame,haijieg/SFrame,TobyRoseman/SFrame,ylow/SFrame,ylow/SFrame,turi-code/SFrame,hoytak/SFrame,haijieg/SFrame,dato-code/SFrame,turi-code/SFrame,TobyRoseman/SFrame,hoytak/SFrame,hoytak/SFrame,hoytak/SFrame,turi-code/SFrame,ylow/SFrame