[SPARK-18448][CORE] Fix @since 2.1.0 on new SparkSession.close() method

## What changes were proposed in this pull request?

Fix since 2.1.0 on new SparkSession.close() method. I goofed in https://github.com/apache/spark/pull/15932

 because it was back-ported to 2.1 instead of just master as originally planned.

Author: Sean Owen <sowen@cloudera.com>

Closes #15938 from srowen/SPARK-18448.2.

(cherry picked from commit ded5fefb)
Signed-off-by: Sean Owen <sowen@cloudera.com>

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

@@ -652,7 +652,7 @@ class SparkSession private(
   /**
    * Synonym for `stop()`.
    *
-   * @since 2.2.0
+   * @since 2.1.0
    */
   override def close(): Unit = stop()