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

## 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`

Author: zero323 <zero323@users.noreply.github.com>

Closes #17779 from zero323/REPEAT-NOTE.