Avoid using the word "public variable section" since it does not exist.

Additionally avoid "formal parameters" as it is unclear as well.
Closes modelica#3437

chapters/functions.tex

@@ -152,7 +152,7 @@ \section{Function as a Specialized Class}\label{function-as-a-specialized-class}
 Modelica \lstinline!class!:
 \begin{itemize}
 \item
-  Only input and output formal parameters are allowed in the function's public variable section.
+  All public components must have prefixes input or output.
 \item
   Input formal parameters are read-only after being bound to the actual
   arguments or default values, i.e., they shall not be assigned values in