[MINOR] Add Sphinx into dev/requirements.txt

[HyukjinKwon]

What changes were proposed in this pull request?

Not a big deal but this PR adds sphinx into dev/requirements.txt since we found it needed - apache/spark-website#122 (comment)

How was this patch tested?

manually:

pip install -r requirements.txt

[HyukjinKwon]

cc @felixcheung

[SparkQA]

Test build #92749 has finished for PR 21735 at commit 038660a.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[HyukjinKwon]

retest this please

[SparkQA]

Test build #92782 has finished for PR 21735 at commit 038660a.

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

[HyukjinKwon]

Merged to master.

[felixcheung]

LGTM thx

[HyukjinKwon]

Thank you @felixcheung.