In Window_Skill, there's a variable called @column_max. You just change the 2 to a 1.
Now this satisfies the request, but I'm sure you're going to ask about formatting next, specifically the SP cost. That would be this line within the same class:
self.contents.draw_text(x + 232, y, 48, 32, skill.sp_cost.to_s, 2)
Change the 232 to a larger number to move it right.
EDIT: Forgot one more thing. In the draw_text method, you also need to change these lines
I just removed the scenario where the second skill would be drawn in the second column.