Rename example file to avoid 404 and also for consistency

[HelderMagalhaes]

Broken link noticed in: https://github.com/jenkinsci/lockable-resources-plugin/blob/master/src/doc/examples/readme.md

Testing done

N/A

Proposed upgrade guidelines

N/A

Localizations

N/A

Submitter checklist

• The Jira / Github issue, if it exists, is well-described.

• The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood (see examples).
  • The changelog generator for plugins uses the pull request title as the changelog entry.
  • Fill in the Proposed upgrade guidelines section only if there are breaking changes or changes that may require extra steps from users during the upgrade.
• There is automated testing or an explanation that explains why this change has no tests.
• New public functions for internal use only are annotated with @NoExternalUse. In case it is used by non java code the Used by {@code <panel>.jelly} Javadocs are annotated.

• New or substantially changed JavaScript is not defined inline and does not call eval to ease the future introduction of Content Security Policy (CSP) directives (see documentation).
• For dependency updates, there are links to external changelogs and, if possible, full differentials.
• For new APIs and extension points, there is a link to at least one consumer.
• Any localizations are transferred to *.properties files.
• Changes in the interface are documented also as examples.

Maintainer checklist

Before the changes are marked as ready-for-merge:

• There is at least one (1) approval for the pull request and no outstanding requests for change.
• Conversations in the pull request are over, or it is explicit that a reviewer is not blocking the change.
• Changelog entries in the pull request title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood.
• Proper changelog labels are set so that the changelog can be generated automatically. See also release-drafter-labels.
• If the change needs additional upgrade steps from users, the upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the pull request title (see example).
• java code changes are tested by automated test.

[MarkEWaite]

Thanks!