Fix an incorrect return type.

Fix #385.

Author: hageboeck

talk/objectorientation/objectsclasses.tex

@@ -79,7 +79,7 @@
       void MyFirstClass::squareA() {
         a *= a;
       }
-      void MyFirstClass::sum(int b) {
+      int MyFirstClass::sum(int b) {
         return a + b;
       }
     \end{cppcode*}