Update cross-key.md

Author: ViktarStarastsenka

src/ds/ts/cross-key.md

@@ -1,7 +1,5 @@
-## Cross-key aggregations
-
 `TS.MRANGE` and `TS.REVMRANGE` can also take an optional `AGGREGATION` argument. This allows you to run queries across different keys using different filters. In the following example, the specified filter looks across time series keys that have a matching `region` label, calculates the daily average using the `AGGREGATION` clause, and groups the data by region and applying the `sum` reducer function.
 
 ```
 TS.MRANGE - + WITHLABELS FILTER region=(east,west) AGGREGATION avg 86400000 GROUPBY region REDUCE sum
-```
+```