fix some struct

example/main.cpp

@@ -1,6 +1,6 @@
 #include "layers/ui_layer/ui_layer.hpp"
 
-namespace sandbox
+namespace example
 {
   class app : public VE::application
   {
@@ -12,10 +12,10 @@ namespace sandbox
 
 auto main(int argc, char* args[]) -> int
 {
-  sandbox::app app{ "vertex_engine_sandbox" };
+  example::app app{ "vertex_engine_sandbox" };
 
   app.push_layer(
-    new sandbox::ui_layer{}
+    new example::ui_layer{}
   );
   app.run();
 }