Fix error in theory

theory/SPH/Derivation/sph_derivation.tex

@@ -186,7 +186,7 @@ The spatial differential is fairly straightforward and is given by
 The differential with respect to the smoothing length is also straightforward
 after remembering that $W_{ij}(h_j) = w(|r_{ij}|/h_j)/h_j^{n_d}$. Then,
 \begin{align}
-  \frac{\partial y_i}{\partial h_i} = -\sum_{j=1}^N \frac{x_j}{h_j}
+  \frac{\partial y_i}{\partial h_i} = -\sum_{j=1}^N \frac{x_j}{h_i}
   \left[
     n_d W_{ij}(h_i) + \frac{|r_{ij}|}{h_i}
     \left.