doc: Fix rendering of slint::interpreter::ComponentInstance::set_global_callback

Don't use a preview for the Slint snippet that just shows a global. There's
nothing to render.

api/cpp/include/slint_interpreter.h

@@ -725,7 +725,7 @@ public:
     /// Like `set_callback()` but on a callback in the specified exported global singleton.
     ///
     /// Example: imagine the .slint file contains the given global:
-    /// ```slint
+    /// ```slint,no-preview
     ///    export global Logic := {
     ///         callback to_uppercase(string) -> string;
     ///    }