Using ground vs virtual ground in a single-supply Sallen-Key low-pass filter
C2 is a capacitor and, providing the "0V" point you connected it to has a low AC impedance and it has low noise compared to the input signals "ground" reference then it doesn't matter. DC doesn't pass through the capacitor to any great extent and any leakage can be reasonably ignored except in the most precision applications.

If the 5V power rail were clean you could connect C2 to that rail.