[MINOR] Avoid the 'latest' link that might vary per release in functions.scala's comment

[HyukjinKwon]

What changes were proposed in this pull request?

This PR propose to address #21318 (comment) comment.

This is rather a nit but looks we better avoid to update the link for each release since it always points the latest (it doesn't look like worth enough updating release guide on the other hand as well).

How was this patch tested?

N/A

[holdensmagicalunicorn]

@HyukjinKwon, thanks! I am a bot who has found some folks who might be able to help with the review:@gatorsmile, @rxin and @davies

[SparkQA]

Test build #93713 has finished for PR 21907 at commit d85d9e0.

• This patch fails due to an unknown error code, -9.
• This patch merges cleanly.
• This patch adds no public classes.

[HyukjinKwon]

retest this please

[SparkQA]

Test build #93717 has finished for PR 21907 at commit d85d9e0.

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

[felixcheung]

probably a good idea. any other web link you want to change?

[HyukjinKwon]

Nope .. this is rather a followup for the PR to address the comment :-). This link particularly contains some contents that frequently changed (list of functions) release by release.

[HyukjinKwon]

Merged to master.

Thank you @felixcheung.