[MINOR][SQL] Fix minor formatting issue of SortAggregateExec.toString

[liancheng]

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.

[SparkQA]

Test build #63173 has finished for PR 14480 at commit 43081c5.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[rxin]

LGTM

[liancheng]

Thanks, merging to master.