Drop stale `.cancel()`, fix docstring typo in tests

- Remove leftover `await an.cancel()` in
  `test_registered_custom_err_relayed`; the
  nursery already cancels on scope exit.
- Fix `This document` -> `This documents` typo in
  `test_unregistered_err_still_relayed` docstring.

Review: PR #426 (Copilot)
https://github.com/goodboy/tractor/pull/426

(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

tests/test_reg_err_types.py

@@ -215,8 +215,6 @@ def test_registered_custom_err_relayed(
                     )
                     raise
 
-            await an.cancel()
-
     with pytest.raises(RemoteActorError) as excinfo:
         trio.run(main)
 
@@ -281,7 +279,7 @@ def test_unregistered_err_still_relayed(
     `.boxed_type_str` and `.src_type_str` still report the original
     type name from the IPC msg.
 
-    This document the expected limitation: without `reg_err_types()`
+    This documents the expected limitation: without `reg_err_types()`
     the `.boxed_type` property can NOT resolve to the original Python
     type.