[SPARK-20101][SQL][FOLLOW-UP] use correct config name "spark.sql.columnVector.offheap.enabled"

## What changes were proposed in this pull request?

This PR addresses [the spelling miss](#17436 (comment)) of the config name `spark.sql.columnVector.offheap.enabled`.
We should use `spark.sql.columnVector.offheap.enabled`.

## How was this patch tested?

Existing tests

Author: Kazuaki Ishizaki <[email protected]>

Closes #19794 from kiszk/SPARK-20101-follow.

Author: kiszk

sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala

@@ -141,7 +141,7 @@ object SQLConf {
       .createWithDefault(true)
 
   val COLUMN_VECTOR_OFFHEAP_ENABLED =
-    buildConf("spark.sql.columnVector.offheap.enable")
+    buildConf("spark.sql.columnVector.offheap.enabled")
       .internal()
       .doc("When true, use OffHeapColumnVector in ColumnarBatch.")
       .booleanConf