[list-view] Correct header s/Grid/BoxLayout
[list-view] Correct header s/Grid/BoxLayout

This classes inherits from MxBoxLayout, not MxGrid. Correct the header
to reflect this.

clutter-project/mx,clutter-project/mx,clutter-project/mx,jonnylamb/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx,jonnylamb/mx,pwithnall/mx,jonnylamb/mx,pwithnall/mx,pwithnall/mx