[MINOR][DOC] Fix python variance() documentation
## What changes were proposed in this pull request?
The Python documentation incorrectly says that `variance()` acts as `var_pop` whereas it acts like `var_samp` here: https://spark.apache.org/docs/latest/api/python/pyspark.sql.html#pyspark.sql.functions.variance
It was not the case in Spark 1.6 doc but it is in Spark 2.0 doc:
https://spark.apache.org/docs/1.6.0/api/java/org/apache/spark/sql/functions.html
https://spark.apache.org/docs/2.0.0/api/java/org/apache/spark/sql/functions.html
The Scala documentation is correct: https://spark.apache.org/docs/latest/api/java/org/apache/spark/sql/functions.html#variance-org.apache.spark.sql.Column-
The alias is set on this line:
https://github.com/apache/spark/blob/v2.4.3/sql/core/src/main/scala/org/apache/spark/sql/functions.scala#L786
## How was this patch tested?
Using variance() in pyspark 2.4.3 returns:
```
>>> spark.createDataFrame([(1, ), (2, ), (3, )], "a: int").select(variance("a")).show()
+-----------+
|var_samp(a)|
+-----------+
| 1.0|
+-----------+
```
Closes #24895 from tools4origins/patch-1.
Authored-by: tools4origins <tools4origins@gmail.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
(cherry picked from commit 25c5d5788311ba74f4aac2f97054b4e9ef7e295c)
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
