[MINOR][SQL] Fix some typos in comments and test hints

## What changes were proposed in this pull request?

Fix some typos in comments and test hints

## How was this patch tested?

N/A.

Author: Sean Zhong <seanzhong@databricks.com>

Closes #14755 from clockfly/fix_minor_typo.

sql/core/src/main/java/org/apache/spark/sql/execution/UnsafeKVExternalSorter.java

@@ -99,7 +99,7 @@ public final class UnsafeKVExternalSorter {
       // The array will be used to do in-place sort, which require half of the space to be empty.
       assert(map.numKeys() <= map.getArray().size() / 2);
       // During spilling, the array in map will not be used, so we can borrow that and use it
-      // as the underline array for in-memory sorter (it's always large enough).
+      // as the underlying array for in-memory sorter (it's always large enough).
       // Since we will not grow the array, it's fine to pass `null` as consumer.
       final UnsafeInMemorySorter inMemSorter = new UnsafeInMemorySorter(
         null, taskMemoryManager, recordComparator, prefixComparator, map.getArray(),

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

@@ -32,9 +32,9 @@ import org.apache.spark.unsafe.KVIterator
  * An iterator used to evaluate aggregate functions. It operates on [[UnsafeRow]]s.
  *
  * This iterator first uses hash-based aggregation to process input rows. It uses
- * a hash map to store groups and their corresponding aggregation buffers. If we
- * this map cannot allocate memory from memory manager, it spill the map into disk
- * and create a new one. After processed all the input, then merge all the spills
+ * a hash map to store groups and their corresponding aggregation buffers. If
+ * this map cannot allocate memory from memory manager, it spills the map into disk
+ * and creates a new one. After processed all the input, then merge all the spills
  * together using external sorter, and do sort-based aggregation.
  *
  * The process has the following step:

sql/core/src/test/scala/org/apache/spark/sql/QueryTest.scala

@@ -358,11 +358,11 @@ abstract class QueryTest extends PlanTest {
    */
   def assertEmptyMissingInput(query: Dataset[_]): Unit = {
     assert(query.queryExecution.analyzed.missingInput.isEmpty,
-      s"The analyzed logical plan has missing inputs: ${query.queryExecution.analyzed}")
+      s"The analyzed logical plan has missing inputs:\n${query.queryExecution.analyzed}")
     assert(query.queryExecution.optimizedPlan.missingInput.isEmpty,
-      s"The optimized logical plan has missing inputs: ${query.queryExecution.optimizedPlan}")
+      s"The optimized logical plan has missing inputs:\n${query.queryExecution.optimizedPlan}")
     assert(query.queryExecution.executedPlan.missingInput.isEmpty,
-      s"The physical plan has missing inputs: ${query.queryExecution.executedPlan}")
+      s"The physical plan has missing inputs:\n${query.queryExecution.executedPlan}")
   }
 }