How do I shave down a sidewalk?
Looking at the flaky condition of the concrete patch and the fact that it is such a small section I'd say the investing time and money in any type of grinding process is a total waste of money. 

The open voids in the surface of the concrete and the grainy texture of the patch area really indicates the low grade of the installed lump.

Break it up, re-form and then pour a new section.

I wouldn't be surprised if you find it very easy to break this up.