Fix white-space errors in recently introduced example on derivative

chapters/functions.tex

@@ -1188,22 +1188,22 @@ \subsection{Using the Derivative Annotation}\label{using-the-derivative-annotati
 The function value, \lstinline!x1!, should up to numerical precision match the integral of the derivative, \lstinline!x2!.
 Note that in practice tools may over-optimize this.
 \begin{lstlisting}[language=modelica]
-  function f
-    input Real x;
-    output Real y;
-    annotation(derivative=fder);
-    external "C";
-  end f;
-  model M
-    input Real u;
-    Real x1 "Directly from function";
-    Real x2 "Integrated from derivative";
-  equation
-    x1 = f(u);
-    der(x2) = der(x1);
-  initial equation
-    x2=x1;
-  end M;
+function f
+  input Real x;
+  output Real y;
+  annotation(derivative=fder);
+  external "C";
+end f;
+model M
+  input Real u;
+  Real x1 "Directly from function";
+  Real x2 "Integrated from derivative";
+equation
+  x1 = f(u);
+  der(x2) = der(x1);
+initial equation
+  x2 = x1;
+end M;
 \end{lstlisting}
 \end{nonnormative}