removed warning from example.cpp

example/example.cpp

@@ -94,7 +94,7 @@ int main(int, char* [])
 // Example of user defined class with operator<<
 //
 class some_class {};
-std::ostream& operator<<(std::ostream& os, const some_class& c) { return os << "some_class"; }
+std::ostream& operator<<(std::ostream& os, const some_class&) { return os << "some_class"; }
 
 void custom_class_example()
 {