[SQL][MINOR] Fix scaladoc for UDFRegistration

## What changes were proposed in this pull request?

Fix scaladoc for UDFRegistration

## How was this patch tested?

local build

Author: Jacek Laskowski <jacek@japila.pl>

Closes #17337 from jaceklaskowski/udfregistration-scaladoc.

(cherry picked from commit 6326d406)
Signed-off-by: Reynold Xin <rxin@databricks.com>

sql/core/src/main/scala/org/apache/spark/sql/UDFRegistration.scala

@@ -36,7 +36,11 @@ import org.apache.spark.sql.types.{DataType, DataTypes}
 import org.apache.spark.util.Utils
 
 /**
- * Functions for registering user-defined functions. Use `SQLContext.udf` to access this.
+ * Functions for registering user-defined functions. Use `SparkSession.udf` to access this:
+ *
+ * {{{
+ *   spark.udf
+ * }}}
  *
  * @note The user-defined functions must be deterministic.
  *