component/catch_event: improve the example for CatchEvent() (#351)

before this change, the example in the doxygen document for CatchEvent()
does not use the documented function. hence the example is not that
useful for developers interested in a typical usage of this function.

after this change, the example is improved to added the call to
CatchEvent(), which allows the application to the Loop() function at
user input.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>

src/ftxui/component/catch_event.cpp

@@ -39,7 +39,14 @@ class CatchEventBase : public ComponentBase {
 /// auto renderer = Renderer([] {
 ///   return text("My interface");
 /// });
-/// screen.Loop(renderer);
+/// auto component = CatchEvent(renderer, [&](Event event) {
+///   if (event == Event::Character('q')) {
+///     screen.ExitLoopClosure()();
+///     return true;
+///   }
+///   return false;
+/// });
+/// screen.Loop(component);
 /// ```
 Component CatchEvent(Component child,
                      std::function<bool(Event event)> on_event) {