Update README.md and example log section

[AlexandreYang]

What does this PR do?

1. Update README.md log section to be more consistent with other integrations.

2. Remove instance and init_config from conf.yaml.example since those are not necessary for a log only integration. It also avoids the agent displaying this error:

  Loading Errors
  ==============
    tenable
    -------
      Core Check Loader:
        Check tenable not found in Catalog

      JMX Check Loader:
        check is not a jmx check, or unable to determine if it's so

      Python Check Loader:
        unable to import module 'tenable': No module named 'tenable'

3. Since this is a logs integration. Let's uncomment the logs section in conf.yaml.example

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

[florimondmanca]

Almost all good — small copy nit.