Fix stale `@pub` docstring example in `experimental`

The `_pubsub.pub` decorator's usage example predates several API
generations: ancient positional-arg-order `run_in_actor()` (a
missing `await` too) plus the deprecated `portal.result()` — and
`run_in_actor()` never allowed streaming funcs anyway. Show the
canonical `start_actor()` + `Portal.open_stream_from()`
consumption instead (#477 removal sweep).

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

tractor/experimental/_pubsub.py

@@ -221,15 +221,18 @@ def pub(
         import tractor
 
         async with tractor.open_nursery() as n:
-            portal = n.run_in_actor(
+            portal = await n.start_actor(
                 'publisher',  # actor name
+                enable_modules=[__name__],
+            )
+            async with portal.open_stream_from(
                 partial(  # func to execute in it
                     pub_service,
                     topics=('clicks', 'users'),
                     task_name='source1',
                 )
-            )
-            async for value in await portal.result():
+            ) as stream:
+                async for value in stream:
                     print(f"Subscriber received {value}")