[SPARK-37511][DOCS][FOLLOW-UP] Fix documentation build warning from TimedeltaIndex

[HyukjinKwon]

What changes were proposed in this pull request?

This PR is a followup of #34657 that adds underline to match with the title.

Why are the changes needed?

To fix the PySpark documentation build warning:

/.../spark/python/docs/source/reference/pyspark.pandas/indexing.rst:340: WARNING: Title underline too short.

TimedeltaIndex
-------------

Does this PR introduce any user-facing change?

No, dev-only.

How was this patch tested?

Manual build of the PySpark documentation.

[HyukjinKwon]

cc @xinrong-databricks can you take a quick look please?

[SparkQA]

Test build #145842 has finished for PR 34775 at commit 4682604.

• 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/50317/

[SparkQA]

Kubernetes integration test status failure
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/50317/

[HyukjinKwon]

Merged to master.