[SPARK-36751][PYTHON][DOCS][FOLLOW-UP] Fix unexpected section title for Examples in docstring

[HyukjinKwon]

What changes were proposed in this pull request?

This PR is a minor followup of #33992 to fix the warnings during PySpark documentation build:

/.../spark/python/pyspark/sql/functions.py:docstring of pyspark.sql.functions.bit_length:19: WARNING: Unexpected section title or transition.

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/.../spark/python/pyspark/sql/functions.py:docstring of pyspark.sql.functions.octet_length:19: WARNING: Unexpected section title or transition.

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We should always have the same length of hyphens with the title.

Why are the changes needed?

To remove warnings during the documentation build and show the HTML pages correctly.

Does this PR introduce any user-facing change?

This is not released yet, and only in master branch. So, no to end users.

How was this patch tested?

Manually built the docs via make clean html at python/docs directory.

[HyukjinKwon]

cc @sarutak and @yoda-mon FYI

[SparkQA]

Test build #143875 has finished for PR 34196 at commit 47f43fc.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/48388/

[SparkQA]

Kubernetes integration test status failure
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/48388/

[sarutak]

Thank you @HyukjinKwon . I noticed the layout of the examples is broken too.
Can we fix them within this PR?

Otherwise, It LGTM.

[HyukjinKwon]

Thanks man!

[SparkQA]

Test build #143881 has finished for PR 34196 at commit a7bf0e0.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/48394/

[sarutak]

@HyukjinKwon Thank you for following up! Merged to master.

[SparkQA]

Kubernetes integration test status failure
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/48394/