Correct name of input to integrand.

Closes modelica#2563

chapters/functions.tex

@@ -512,7 +512,7 @@ \subsection{Functional Input Arguments to Functions}\doublelabel{functional-inpu
 end quadrature;
 
 partial function Integrand
-  input Real x;
+  input Real u;
   output Real y;
 end Integrand;
 \end{lstlisting}