[SPARK-36541][DOCS][PYTHON] Replace the word Koalas to pandas-on-Spark

### What changes were proposed in this pull request?

Replace images in pyspark on pandas document because those images uses the word Koalas

### Why are the changes needed?

Images in Transform and apply a function documentation still uses the word Koalas, althogh the word was replaced to panas-on-Spark by this PR .
https://github.com/apache/spark/pull/32835

I think we have to match the word on that images

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

`make html`

Screen shots
![130179112-8485fdde-b422-4834-8b23-fe69e7402118](https://user-images.githubusercontent.com/14937752/130186051-d6ff65f0-c121-40bd-b4f1-2fbc10e76f3e.png)
![130179239-8dae7812-4d81-4f8c-8558-b75e4eae3787](https://user-images.githubusercontent.com/14937752/130186063-17d4a95f-0b9d-49d3-85c7-13ea07e4b6bb.png)
![130179273-10f9fbc3-0a62-4e1a-ab6e-7049d75653a1](https://user-images.githubusercontent.com/14937752/130186074-7d684669-b9ef-4a4e-8a2d-c63bb9800ddb.png)
![130179311-616545af-dde2-4dec-807f-dde0a0d4bfbe](https://user-images.githubusercontent.com/14937752/130186095-20669673-b1d3-4552-97bf-86bbc1a5d43b.png)
Environment
- Windows 10
- Google Chrome 92.0.4515.159

[images.pptx](https://github.com/apache/spark/files/7029087/images.pptx)

Closes #33786 from yoda-mon/replace-pyspark-doc-images.

Authored-by: Leona Yoda <yodal@oss.nttdata.com>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>
(cherry picked from commit aeb3da2798)
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

python/docs/source/user_guide/pandas_on_spark/transform_apply.rst

@@ -36,7 +36,7 @@ to return the same length of the input and the latter does not require this. See
 
 In this case, each function takes a pandas Series, and pandas API on Spark computes the functions in a distributed manner as below.
 
-.. image:: https://user-images.githubusercontent.com/6477701/80076790-a1cf0680-8587-11ea-8b08-8dc694071ba0.png
+.. image:: ../../../../../docs/img/pyspark-pandas_on_spark-transform_apply1.png
   :alt: transform and apply
   :align: center
   :width: 550
@@ -53,7 +53,7 @@ In case of 'column' axis, the function takes each row as a pandas Series.
 
 The example above calculates the summation of each row as a pandas Series. See below:
 
-.. image:: https://user-images.githubusercontent.com/6477701/80076898-c2975c00-8587-11ea-9b2c-69c9729e9294.png
+.. image:: ../../../../../docs/img/pyspark-pandas_on_spark-transform_apply2.png
   :alt: apply axis
   :align: center
   :width: 600
@@ -95,7 +95,7 @@ you can avoid a shuffle by the operations between different DataFrames. In case
 treated that it belongs to a new different DataFrame. See also
 `Operations on different DataFrames <options.rst#operations-on-different-dataframes>`_ for more details.
 
-.. image:: https://user-images.githubusercontent.com/6477701/80076779-9f6cac80-8587-11ea-8c92-07d7b992733b.png
+.. image:: ../../../../../docs/img/pyspark-pandas_on_spark-transform_apply3.png
   :alt: pandas_on_spark.transform_batch and pandas_on_spark.apply_batch in Frame
   :align: center
   :width: 650
@@ -113,7 +113,7 @@ a pandas Series as a chunk of pandas-on-Spark Series.
 
 Under the hood, each batch of pandas-on-Spark Series is split to multiple pandas Series, and each function computes on that as below:
 
-.. image:: https://user-images.githubusercontent.com/6477701/80076795-a3003380-8587-11ea-8b73-186e4047f8c0.png
+.. image:: ../../../../../docs/img/pyspark-pandas_on_spark-transform_apply4.png
   :alt: pandas_on_spark.transform_batch in Series
   :width: 350
   :align: center