[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.