minor documentation fix for Shan-Chen

This sum counts over the number of lattice vectors, which is not necessarily (and not in this 2-dimensional test) 19.

doc/notebooks/07_tutorial_shanchen_twophase.ipynb

 The force on the fluid is
-$\vec{F}_A(\vec{x})=-\psi(\rho_A(\vec{x}))g_{AA}\sum\limits_{i=1}^{19}w_i\psi(\rho_A(\vec{x}+\vec{c}_i))\vec{c}_i$
+$\vec{F}_A(\vec{x})=-\psi(\rho_A(\vec{x}))g_{AA}\sum\limits_{i=1}^{q}w_i\psi(\rho_A(\vec{x}+\vec{c}_i))\vec{c}_i$
 with
 $\psi(\rho)=\rho_0\left[1-\exp(-\rho/\rho_0)\right]$.