Issue 2437: Fix typo in ledgers data directory for Docker

[rdehouss]

Fix typo for ledgers directory

Descriptions of the changes in this PR:

Motivation

Typo in the documentation that can leads to issue (data not persistent across docker restarts)

Changes

Fix typo (added the s to ledgers)

Master Issue: #2437

---

In order to uphold a high standard for quality for code contributions, Apache BookKeeper runs various precommit
checks for pull requests. A pull request can only be merged when it passes precommit checks.

---

Be sure to do all of the following to help us incorporate your contribution
quickly and easily:

If this PR is a BookKeeper Proposal (BP):

• Make sure the PR title is formatted like:
  <BP-#>: Description of bookkeeper proposal
e.g. BP-1: 64 bits ledger is support
• Attach the master issue link in the description of this PR.
• Attach the google doc link if the BP is written in Google Doc.

Otherwise:

• Make sure the PR title is formatted like:
  <Issue #>: Description of pull request
e.g. Issue 123: Description ...
• Make sure tests pass via mvn clean apache-rat:check install spotbugs:check.
• Replace <Issue #> in the title with the actual Issue number.

---

[eolivelli]

LGTM

thank you for your quickPR