[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)apache#89,min(c)apache#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)apache#89, min(c)apache#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 apache#14480 from liancheng/fix-sort-based-agg-string-format.

sql/core/src/main/scala/org/apache/spark/sql/execution/aggregate/SortAggregateExec.scala

@@ -111,9 +111,9 @@ case class SortAggregateExec(
   private def toString(verbose: Boolean): String = {
     val allAggregateExpressions = aggregateExpressions
 
-    val keyString = Utils.truncatedString(groupingExpressions, "[", ",", "]")
-    val functionString = Utils.truncatedString(allAggregateExpressions, "[", ",", "]")
-    val outputString = Utils.truncatedString(output, "[", ",", "]")
+    val keyString = Utils.truncatedString(groupingExpressions, "[", ", ", "]")
+    val functionString = Utils.truncatedString(allAggregateExpressions, "[", ", ", "]")
+    val outputString = Utils.truncatedString(output, "[", ", ", "]")
     if (verbose) {
       s"SortAggregate(key=$keyString, functions=$functionString, output=$outputString)"
     } else {