Update 4_ZPFs.md - typo

src/4_ZPFs.md

@@ -61,7 +61,7 @@ Imagine an empty box that contains electromagnetic radiation. Classically, each
 Associated with every electric field $\vec{E}$, there is a magnetic field $\vec{B}$ that oscillates out of phase with respect to the electric field (this follows from Maxwell's Equations).  The total energy is given by:
 
 $$
-E = \int (\epsilon_0 |\vec{E}|^2 + \mu_0 |\vec{B}|^2) d^3 x
+E = \frac{1}{2}\int (\epsilon_0 |\vec{E}|^2 + \mu_0 |\vec{B}|^2) d^3 x
 $$
 
 For a given mode, this will be proportional to: