[embind] Fix warning in example. [ci skip] (emscripten-core#22746)

Fixes the warning "delete called on 'Interface' that is abstract but has
non-virtual destructor"

Author: brendandahl

site/source/docs/porting/connecting_cpp_and_javascript/embind.rst

@@ -597,6 +597,7 @@ implemented in JavaScript.
 .. code:: cpp
 
     struct Interface {
+        virtual ~Interface() {}
         virtual void invoke(const std::string& str) = 0;
     };