Update the README files under scalardb and scalardl directories

[komamitsu]

Description

I noticed there is room to improve the README files under scalardb and scalardl directories. This PR updates the files.

Related issues and/or PRs

None

Changes made

• Add missing necessary option --ssh-private-key option to the example command in scalardb/README.md
  • Without the option, I failed to run the Jepsen test
  • (Should I add it to scalardl/README.md as well?)
• Replace Scalar DB with ScalarDB and Scalar DL with ScalarDL

Checklist

The following is a best-effort checklist. If any items in this checklist are not applicable to this PR or are dependent on other, unmerged PRs, please still mark the checkboxes after you have read and understood each item.

• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation to reflect the changes.
• Any remaining open issues linked to this PR are documented and up-to-date (Jira, GitHub, etc.).
• Tests (unit, integration, etc.) have been added for the changes.
• My changes generate no new warnings.
• Any dependent changes in other PRs have been merged and published.

Additional notes (optional)

None

[yito88]

Thank you!

[brfrn169]

LGTM! Thank you!

[feeblefakie]

LGTM! Thank you!