sql: fix a typo (pingcap#14879)

explain-index-merge.md

@@ -50,7 +50,7 @@ For the preceding query, the optimizer chooses the union-type index merge to acc
 
 In the output, the `type: union` information in `operator info` of the `IndexMerge_8` operator indicates that this operator is a union-type index merge. It has three child nodes. `IndexRangeScan_5` and `IndexRangeScan_6` scan the `RowID`s that meet the condition according to the range, and then the `TableRowIDScan_7` operator accurately reads all the data that meets the condition according to these `RowID`s.
 
-For the scan operation that is performed on a specific range of data, such as `IndexRangeScan`/`TableRangeScan`, the `operator info` column in the result has additional information about the scan range compared with other scan operations like `IndexFullScan`/`TableFullScan`. In the above example, the `range:(1,+inf]` in the `IndexRangeScan_13` operator indicates that the operator scans the data from 1 to positive infinity.
+For the scan operation that is performed on a specific range of data, such as `IndexRangeScan`/`TableRangeScan`, the `operator info` column in the result has additional information about the scan range compared with other scan operations like `IndexFullScan`/`TableFullScan`. In the above example, the `range:(1,+inf]` in the `IndexRangeScan_5` operator indicates that the operator scans the data from 1 to positive infinity.
 
 ```sql
 EXPLAIN SELECT /*+ NO_INDEX_MERGE() */ * FROM t WHERE a > 1 AND b > 1 AND c = 1;  -- Does not use index merge