typo in equation

doc/src/Eqs/fix_rhok.tex

@@ -4,7 +4,7 @@
 
 \begin{eqnarray*}
  U &=&  \frac{1}{2} K (|\rho_{\vec{k}}| - a)^2 \\
- \rho_{\vec{k}} &=& \sum_i^N \exp(-i\vec{k} \cdot \vec{r}_i )/\sqrt{N} \\
+ \rho_{\vec{k}} &=& \sum_j^N \exp(-i\vec{k} \cdot \vec{r}_j )/\sqrt{N} \\
  \vec{k} &=& (2\pi n_x /L_x , 2\pi n_y  /L_y , 2\pi n_z/L_z ) 
 \end{eqnarray*}