[DOCS][MINOR] Add missing since to SparkR repeat_string note.

[zero323]

What changes were proposed in this pull request?

Replace

@note repeat_string 2.3.0

with

@note repeat_string since 2.3.0

How was this patch tested?

create-docs.sh

[SparkQA]

Test build #76212 has finished for PR 17779 at commit e5c1415.

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

[felixcheung]

merged to master

[zero323]

Thanks, and sorry for missing this before.