[SPARK-28786][DOC][SQL][FOLLOW-UP] Change "Related Statements" to bold

[huaxingao]

What changes were proposed in this pull request?

Change "Related Statements" to bold

Why are the changes needed?

To make doc look nice and consistent.

Does this PR introduce any user-facing change?

Yes

How was this patch tested?

Tested using jykyll build --serve

Before the change:

After the change:

Before the change:

After the change:

[huaxingao]

@gatorsmile
One minor follow-up change. Thank you very much in advance!

[SparkQA]

Test build #109915 has finished for PR 25623 at commit 771a236.

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

[SparkQA]

Test build #109934 has finished for PR 25623 at commit ac0b9e9.

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

[SparkQA]

Test build #109961 has finished for PR 25623 at commit 37e2f5f.

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

[dilipbiswal]

LGTM

[gatorsmile]

Thanks! Merged to master.

[huaxingao]

Thanks all for your help! @srowen @dilipbiswal @gatorsmile