[MINOR][SQL] Fix minor formatting issue of SortAggregateExec.toString
## What changes were proposed in this pull request?
This PR fixes a minor formatting issue (missing space after comma) of `SorgAggregateExec.toString`.
Before:
```
SortAggregate(key=[a#76,b#77], functions=[max(c#78),min(c#78)], output=[a#76,b#77,max(c)#89,min(c)#90])
+- *Sort [a#76 ASC, b#77 ASC], false, 0
+- Exchange hashpartitioning(a#76, b#77, 200)
+- SortAggregate(key=[a#76,b#77], functions=[partial_max(c#78),partial_min(c#78)], output=[a#76,b#77,max#99,min#100])
+- *Sort [a#76 ASC, b#77 ASC], false, 0
+- LocalTableScan <empty>, [a#76, b#77, c#78]
```
After:
```
SortAggregate(key=[a#76, b#77], functions=[max(c#78), min(c#78)], output=[a#76, b#77, max(c)#89, min(c)#90])
+- *Sort [a#76 ASC, b#77 ASC], false, 0
+- Exchange hashpartitioning(a#76, b#77, 200)
+- SortAggregate(key=[a#76, b#77], functions=[partial_max(c#78), partial_min(c#78)], output=[a#76, b#77, max#99, min#100])
+- *Sort [a#76 ASC, b#77 ASC], false, 0
+- LocalTableScan <empty>, [a#76, b#77, c#78]
```
## How was this patch tested?
Manually tested.
Author: Cheng Lian <lian@databricks.com>
Closes #14480 from liancheng/fix-sort-based-agg-string-format.
780c7224a5