fix math error hopefully for real

src/8_differential_equations_2.md

@@ -272,7 +272,7 @@ derivative.
 
 For the Schrödinger equation, we could supply the initial conditions
 
-$$\psi(x,0)= \psi_{0}(x) \ & \ \psi(0,t) = \psi(L, t) = 0 \text{for all $t$}.$$
+$$\psi(x,0)= \psi_0(x) \& \ \psi(0,t) = \psi(L, t) = 0 \text{for all $t$}.$$
 
 This particular set of boundary conditions corresponds to a particle in a box,
 a situation which is used as the base model for many derivations in quantum