fix: replace unsupported MATERIALIZED VIEW with AGGREGATING INDEX (#2741)

- Replace CREATE MATERIALIZED VIEW syntax with CREATE AGGREGATING INDEX
- Update Advanced Optimization section to use Databend's native aggregating index feature
- Ensure documentation accurately reflects supported functionality

Author: BohuTANG

docs/en/guides/54-query/03-optimization/index.md

@@ -171,10 +171,10 @@ LIMIT 10;
 
 ## Advanced Optimization
 
-### Materialized Views
+### Aggregating Indexes
 ```sql
--- Pre-compute expensive aggregations
-CREATE MATERIALIZED VIEW daily_sales AS
+-- Pre-compute expensive aggregations using Databend's aggregating indexes
+CREATE AGGREGATING INDEX daily_sales_agg AS
 SELECT 
     DATE(order_time) as order_date,
     product_id,