[MINOR] Minor comment fixes in merge_spark_pr.py script

## What changes were proposed in this pull request?

This PR proposes to fix few rather typos in `merge_spark_pr.py`.

- `#   usage: ./apache-pr-merge.py    (see config env vars below)`
  -> `#   usage: ./merge_spark_pr.py    (see config env vars below)`

- `... have local a Spark ...` -> `... have a local Spark ...`

- `... to Apache.` -> `... to Apache Spark.`

I skimmed this file and these look all I could find.

## How was this patch tested?

pep8 check (`./dev/lint-python`).

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #18776 from HyukjinKwon/minor-merge-script.