[SPARKR][MINOR] Fix windowPartitionBy example

## What changes were proposed in this pull request?

The usage in the original example is incorrect. This PR fixes it.

## How was this patch tested?

Manual test.

Author: Junyang Qian <junyangq@databricks.com>

Closes #14903 from junyangq/SPARKR-FixWindowPartitionByDoc.

R/pkg/R/window.R

@@ -21,9 +21,9 @@
 #'
 #' Creates a WindowSpec with the partitioning defined.
 #'
-#' @param col A column name or Column by which rows are partitioned to 
+#' @param col A column name or Column by which rows are partitioned to
 #'            windows.
-#' @param ... Optional column names or Columns in addition to col, by 
+#' @param ... Optional column names or Columns in addition to col, by
 #'            which rows are partitioned to windows.
 #'
 #' @rdname windowPartitionBy
@@ -32,10 +32,10 @@
 #' @export
 #' @examples
 #' \dontrun{
-#'   ws <- windowPartitionBy("key1", "key2")
+#'   ws <- orderBy(windowPartitionBy("key1", "key2"), "key3")
 #'   df1 <- select(df, over(lead("value", 1), ws))
 #'
-#'   ws <- windowPartitionBy(df$key1, df$key2)
+#'   ws <- orderBy(windowPartitionBy(df$key1, df$key2), df$key3)
 #'   df1 <- select(df, over(lead("value", 1), ws))
 #' }
 #' @note windowPartitionBy(character) since 2.0.0
@@ -70,9 +70,9 @@ setMethod("windowPartitionBy",
 #'
 #' Creates a WindowSpec with the ordering defined.
 #'
-#' @param col A column name or Column by which rows are ordered within 
+#' @param col A column name or Column by which rows are ordered within
 #'            windows.
-#' @param ... Optional column names or Columns in addition to col, by 
+#' @param ... Optional column names or Columns in addition to col, by
 #'            which rows are ordered within windows.
 #'
 #' @rdname windowOrderBy