update userguide (#15710)

Author: getChan

docs/source/user-guide/sql/explain.md

@@ -71,7 +71,7 @@ to see the high level structure of the plan
 | | ┌─────────────┴─────────────┐ |
 | | │ RepartitionExec │ |
 | | │ -------------------- │ |
-| | │ output_partition_count: │ |
+| | │  input_partition_count: │ |
 | | │ 16 │ |
 | | │ │ |
 | | │ partitioning_scheme: │ |
@@ -80,7 +80,7 @@ to see the high level structure of the plan
 | | ┌─────────────┴─────────────┐ |
 | | │ RepartitionExec │ |
 | | │ -------------------- │ |
-| | │ output_partition_count: │ |
+| | │  input_partition_count: │ |
 | | │ 1 │ |
 | | │ │ |
 | | │ partitioning_scheme: │ |