Fix half-baked partial derivative as suggestede

Co-authored-by: Hans Olsson <[email protected]>

chapters/operatorsandexpressions.tex

@@ -965,7 +965,7 @@ \subsubsection{spatialDistribution}\label{spatialdistribution}
 
 \lstinline!spatialDistribution! allows the infinite-dimensional problem below to be solved efficiently with good accuracy
 \begin{align*}
-\frac{\partial z(\xi, t)}{\partial t} + v(t) \frac{\partial z(\xi, t)}{\partial x} &= 0\\
+\frac{\partial z(\xi, t)}{\partial t} + v(t) \frac{\partial z(\xi, t)}{\partial \xi} &= 0\\
 z(0, t) &= \mathrm{in}_{0}(t) \quad \text{if $v \geq 0$}\\
 z(1, t) &= \mathrm{in}_{1}(t) \quad \text{if $v < 0$}
 \end{align*}