[SPARK-40142][PYTHON][DOCS][FOLLOW-UP] Remove non-ANSI compliant example in element_at

[HyukjinKwon]

What changes were proposed in this pull request?

This PR is a followup of #37850 that removes non-ANSI compliant example in element_at.

Why are the changes needed?

ANSI build fails to run the example.

https://github.com/apache/spark/actions/runs/3094607589/jobs/5008176959

    Caused by: org.apache.spark.SparkArrayIndexOutOfBoundsException: [INVALID_ARRAY_INDEX_IN_ELEMENT_AT] The index -4 is out of bounds. The array has 3 elements. Use `try_element_at` to tolerate accessing element at invalid index and return NULL instead. If necessary set "spark.sql.ansi.enabled" to "false" to bypass this error.
    	at org.apache.spark.sql.errors.QueryExecutionErrors$.invalidElementAtIndexError(QueryExecutionErrors.scala:264)
    	...

/usr/local/pypy/pypy3.7/lib-python/3/runpy.py:125: RuntimeWarning: 'pyspark.sql.functions' found in sys.modules after import of package 'pyspark.sql', but prior to execution of 'pyspark.sql.functions'; this may result in unpredictable behaviour
  warn(RuntimeWarning(msg))
/__w/spark/spark/python/pyspark/context.py:310: FutureWarning: Python 3.7 support is deprecated in Spark 3.4.
  warnings.warn("Python 3.7 support is deprecated in Spark 3.4.", FutureWarning)
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   1 of   6 in pyspark.sql.functions.element_at

Does this PR introduce any user-facing change?

No. The example added is not exposed to end users yet.

How was this patch tested?

Manually tested with enabling the ANSI configuration (spark.sql.ansi.enabled)

[HyukjinKwon]

@gengliangwang mind taking a quick look please? 🙏

[itholic]

LGTM

[HyukjinKwon]

Merged to master.