xml::Optional example added end extended

doc/examples/optional_serialization.cxx

@@ -37,7 +37,7 @@ class B: public xml::Serialize {
 
 int main(int, char**) {
   { // Serialization as a member
-    std::stringstream ss("<B/>");
+    std::stringstream ss("<B><a><txt>Only this text is given</txt></a></B>");
     B b;
     b.loadXml(ss);
     b.saveXml(std::cout)<<std::endl;