A question about a recipe: Chocolate Bundt Cake

I have a question about step 3 on the recipe "Chocolate Bundt Cake" from KelseyTheNaptimeChef. It says:

"In a mixer on low add the milk, coffee, vegetable oil, eggs and vanilla one at a time. mix until everything is incorporated. Then, with the mixer still on low speed, slowly add in the dry ingredients. Once all of the flour mixture is added, mix the batter for a full four minutes on medium speed."

how do i make "sour" milk?