fix unexpected indentation in Client.cancel docstring

[graingert]

running sphinx with -W fails with:

site-packages/distributed/client.py:docstring of distributed.client.Client.cancel:9:Unexpected indentation.

and results in ugly output https://docs.dask.org/en/latest/futures.html#distributed.Client.cancel

before:

after:

• Tests added / passed
• Passes pre-commit run --all-files

[github-actions]

Unit Test Results

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

     26 files  ±0       26 suites  ±0   1h 16m 28s ⏱️ -15s
3 143 tests ±0  2 929 ✔️ ±0  211 💤  - 1  2 ❌ +1    1 🔥 ±0 
6 424 runs  ±0  5 623 ✔️ ±0  777 💤  - 1  2 ❌ +1  22 🔥 ±0 

For more details on these failures and errors, see this check.

Results for commit c2b603d. ± Comparison against base commit 6fb7f0e.

[graingert]

there are more warnings that sphinx prints, and I've fixed the rest of them here #7697 (and enabled warning checking) but this is the only warning that has a visual impact

[jrbourbeau]

Thanks @graingert!