docs: fix Scheduler.close docstring

[cnaples79]

Summary

• remove reference to non-existent Scheduler.cleanup and tighten Scheduler.close docstring

Rationale

• docstring pointed to a method that doesn't exist (issue Docstring for Scheduler.close refers to non-existent Scheduler.cleanup method #9175)

Changes

• distributed/scheduler.py: docstring now accurately describes close

Tests

• not run (docs-only)

Closes #9175

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

    31 files  ±0      31 suites  ±0   11h 21m 11s ⏱️ +17s
 4 113 tests ±0   4 002 ✅  - 4    104 💤 ±0  7 ❌ +4 
59 636 runs  ±0  57 155 ✅  - 4  2 474 💤 ±0  7 ❌ +4 

For more details on these failures, see this check.

Results for commit 942b772. ± Comparison against base commit 4fb4814.