doc: fix from_sync_fn using deprecated Result<T> return

Author: arocull

godot-core/src/builtin/callable.rs

@@ -282,7 +282,7 @@ impl Callable {
     /// # use godot::prelude::*;
     /// let callable = Callable::from_sync_fn("sum", |args: &[&Variant]| {
     ///     let sum: i32 = args.iter().map(|arg| arg.to::<i32>()).sum();
-    ///     Ok(sum.to_variant())
+    ///     sum
     /// });
     /// ```
     #[cfg(feature = "experimental-threads")]