information in README.md is inaccurate

[shaoyongfeng]

Describe the bug In README.md, there is a option called --elasticsearch for logs sub command. but it doesn't support at all.

To Reproduce Steps to reproduce the behavior: run gardenctl logs etcd-main --elasticsearch

Expected behavior please adjust README.md. based on my testing, correct command should be gardenctl logs etcd-main etcd

Screenshots If applicable, add screenshots to help explain your problem.

Gardenctl Version (please complete the following information):

• Version/Commit: latest

Additional context Add any other context about the problem here.