[doc] Fix error in handbook

In the section where the assembly of the linear system is explained,
the RHS of the linear system should be the residual r(u) and not the
primary variable u itself. It should read J(u)*x=r(u).

doc/handbook/4_assemblinglinearsystem.tex

@@ -38,7 +38,7 @@ Each column is the residual derived with respect to the $m$th entry of $\textbf{
 The Jacobian indicates the direction where the residual increases. By solving the
 linear system
 \begin{align*}
-  J_{\textbf{r}(\textbf{u}^i)} \cdot \textbf{x}^i = \textbf{u}^i
+  J_{\textbf{r}(\textbf{u}^i)} \cdot \textbf{x}^i = \textbf{r}(\textbf{u}^i)
 \end{align*}
 we calculate the direction of maximum growth $\textbf{x}^i$. We subtract it from
 our current solution to get a new, better solution