Update doc config vrf (backport #1538)

[mergify]

Change Summary

Remove example without id. VyOS does not accept this example.
See https://forum.vyos.io/t/vrf-table-id-is-mandatory/13685

Backport

Checklist:

• I have read the CONTRIBUTING document

  ---

  This is an automatic backport of pull request Update doc config vrf #1538 done by Mergify.

[mergify]

Cherry-pick of 63fb97b has failed:

On branch mergify/bp/current/pr-1538
Your branch is up to date with 'origin/current'.

You are currently cherry-picking commit 63fb97b2.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   docs/configuration/vrf/index.rst

no changes added to commit (use "git add" and/or "git commit -a")

Cherry-pick of d70a4e2 has failed:

On branch mergify/bp/current/pr-1538
Your branch is ahead of 'origin/current' by 1 commit.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit d70a4e22.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   docs/configuration/vrf/index.rst

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally