[SPARK-26451][SQL] Change lead/lag argument name from count to offset

## What changes were proposed in this pull request?

Change aligns argument name with that in Scala version and documentation.

## How was this patch tested?

(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)
(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)

Please review http://spark.apache.org/contributing.html before opening a pull request.

Closes #23357 from deepyaman/patch-1.

Authored-by: deepyaman <deepyaman.datta@utexas.edu>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

python/pyspark/sql/functions.py

@@ -798,7 +798,7 @@ def factorial(col):
 # ---------------  Window functions ------------------------
 
 @since(1.4)
-def lag(col, count=1, default=None):
+def lag(col, offset=1, default=None):
     """
     Window function: returns the value that is `offset` rows before the current row, and
     `defaultValue` if there is less than `offset` rows before the current row. For example,
@@ -807,15 +807,15 @@ def lag(col, count=1, default=None):
     This is equivalent to the LAG function in SQL.
 
     :param col: name of column or expression
-    :param count: number of row to extend
+    :param offset: number of row to extend
     :param default: default value
     """
     sc = SparkContext._active_spark_context
-    return Column(sc._jvm.functions.lag(_to_java_column(col), count, default))
+    return Column(sc._jvm.functions.lag(_to_java_column(col), offset, default))
 
 
 @since(1.4)
-def lead(col, count=1, default=None):
+def lead(col, offset=1, default=None):
     """
     Window function: returns the value that is `offset` rows after the current row, and
     `defaultValue` if there is less than `offset` rows after the current row. For example,
@@ -824,11 +824,11 @@ def lead(col, count=1, default=None):
     This is equivalent to the LEAD function in SQL.
 
     :param col: name of column or expression
-    :param count: number of row to extend
+    :param offset: number of row to extend
     :param default: default value
     """
     sc = SparkContext._active_spark_context
-    return Column(sc._jvm.functions.lead(_to_java_column(col), count, default))
+    return Column(sc._jvm.functions.lead(_to_java_column(col), offset, default))
 
 
 @since(1.4)