[DOCS] Refactored Dataframe join comment to use correct parameter ordering

The API signatire for join requires the JoinType to be the third parameter. The code examples provided for join show JoinType being provided as the 2nd parater resuling in errors (i.e. "df1.join(df2, "outer", $"df1Key" === $"df2Key") ). The correct sample code is df1.join(df2, $"df1Key" === $"df2Key", "outer")

Author: Paul Power <paul.power@peerside.com>

Closes #4847 from peerside/master and squashes the following commits:

ebc1efa [Paul Power] Merge pull request #1 from peerside/peerside-patch-1
e353340 [Paul Power] Updated comments use correct sample code for Dataframe joins

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

@@ -337,11 +337,11 @@ class DataFrame protected[sql](
    * {{{
    *   // Scala:
    *   import org.apache.spark.sql.functions._
-   *   df1.join(df2, "outer", $"df1Key" === $"df2Key")
+   *   df1.join(df2, $"df1Key" === $"df2Key", "outer")
    *
    *   // Java:
    *   import static org.apache.spark.sql.functions.*;
-   *   df1.join(df2, "outer", col("df1Key") === col("df2Key"));
+   *   df1.join(df2, col("df1Key").equalTo(col("df2Key")), "outer");
    * }}}
    *
    * @param right Right side of the join.