chore: fix return_field_from_args doc (apache#19307)

## Which issue does this PR close?

- Closes apache#19306

## Rationale for this change

N/A

## What changes are included in this PR?

Make use of the calculated `nullable`.

## Are these changes tested?

Manual test.

## Are there any user-facing changes?

No.

Author: xumingming

datafusion/expr/src/udf.rs

@@ -610,7 +610,7 @@ pub trait ScalarUDFImpl: Debug + DynEq + DynHash + Send + Sync {
     /// fn return_field_from_args(&self, args: ReturnFieldArgs) -> Result<FieldRef> {
     ///     // report output is only nullable if any one of the arguments are nullable
     ///     let nullable = args.arg_fields.iter().any(|f| f.is_nullable());
-    ///     let field = Arc::new(Field::new("ignored_name", DataType::Int32, true));
+    ///     let field = Arc::new(Field::new("ignored_name", DataType::Int32, nullable));
     ///     Ok(field)
     /// }
     /// # }