[temporal discretization]

added "a"

doc/handbook/5_models.tex

@@ -241,7 +241,7 @@ whereas the implicit Euler method is described as
 Once the solution $u_k$ at time $t_k$ is known, it is straightforward
 to determine $m(u_{k+1})$ from \eqref{eq:expliciteuler},
 while attempting to do the same based on \eqref{eq:impliciteuler}
-involves the solution of system of equations.
+involves the solution of a system of equations.
 On the other hand, the explicit method \eqref{eq:expliciteuler} is stable only
 if the time step size $\Delta t_{k+1}$ is below a certain limit that depends
 on the specific balance equation, whereas the implicit method \eqref{eq:impliciteuler}