[MINOR] [SQL] Update usage of `debug` by removing `typeCheck` and adding `debugCodegen`

## What changes were proposed in this pull request?

This PR updates the usage comments of `debug` according to the following commits.
- [SPARK-9754](https://issues.apache.org/jira/browse/SPARK-9754) removed `typeCheck`.
- [SPARK-14227](https://issues.apache.org/jira/browse/SPARK-14227) added `debugCodegen`.

## How was this patch tested?

Manual.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #12094 from dongjoon-hyun/minor_fix_debug_usage.

sql/core/src/main/scala/org/apache/spark/sql/execution/debug/package.scala

@@ -35,8 +35,8 @@ import org.apache.spark.sql.internal.SQLConf
  * Usage:
  * {{{
  *   import org.apache.spark.sql.execution.debug._
- *   sql("SELECT key FROM src").debug()
- *   dataFrame.typeCheck()
+ *   sql("SELECT 1").debug()
+ *   sql("SELECT 1").debugCodegen()
  * }}}
  */
 package object debug {