chore(query): update comment in rule_grouping_sets_to_union.rs (#18450)

* [chore] update comment in rule_grouping_sets_to_union.rs

* Addressed

---------

Co-authored-by: sundyli <[email protected]>

Author: xiedeyantu

src/query/sql/src/planner/optimizer/optimizers/rule/agg_rules/rule_grouping_sets_to_union.rs

@@ -42,7 +42,6 @@ use crate::plans::VisitorMut;
 use crate::IndexType;
 use crate::ScalarExpr;
 
-// TODO
 const ID: RuleID = RuleID::GroupingSetsToUnion;
 // Split `Grouping Sets` into `Union All` of `Group by`
 // Eg:
@@ -52,11 +51,11 @@ const ID: RuleID = RuleID::GroupingSetsToUnion;
 
 // INTO:
 
-// select number % 10 AS a, number % 3 AS b, number % 4 AS c
+// select number % 10 AS a, number % 3 AS b, null AS c
 // from numbers(100000000)
 // group by a,b
 // union all
-// select number % 10 AS a, number % 3 AS b, number % 4 AS c
+// select number % 10 AS a, null AS b, number % 4 AS c
 // from numbers(100000000)
 // group by a,c
 //