fix(docs): remove incorrect flags for scrubber purge-garbage command

[yliang412]

Scrubber purge-garbage command does not take --node-kind and --depth.

[github-actions]

3126 tests run: 3005 passed, 0 failed, 121 skipped (full report)

---

Flaky tests (4)

Postgres 16

• test_change_pageserver: debug

Postgres 15

• test_change_pageserver: debug

Postgres 14

• test_change_pageserver: debug
• test_scrubber_physical_gc_ancestors[2]: debug

Code coverage* (full report)

• functions: 32.6% (7003 of 21480 functions)
• lines: 50.0% (55259 of 110560 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
04468e8 at 2024-07-22T18:07:30.069Z :recycle:}