Pass `pub_service` directly to stream RPC

Keep the `@pub` docstring example's remote target namespace
addressable by passing the module-level function directly to
`Portal.open_stream_from()`.

Forward the topic and task-name inputs as RPC kwargs instead of
wrapping the target in a `functools.partial` object that resolves to
the wrong namespace path.

Review: PR #484 (OpenCode)
https://github.com/goodboy/tractor/pull/484#pullrequestreview-5025383596

(this patch was generated in some part by `opencode` using `gpt-5.6-sol` (`openai`))

tractor/experimental/_pubsub.py

@@ -217,7 +217,6 @@ def pub(
 
     .. code:: python
 
-        from functools import partial
         import tractor
 
         async with tractor.open_nursery() as n:
@@ -226,11 +225,9 @@ def pub(
                 enable_modules=[__name__],
             )
             async with portal.open_stream_from(
-                partial(  # func to execute in it
-                    pub_service,
-                    topics=('clicks', 'users'),
-                    task_name='source1',
-                )
+                pub_service,  # func to execute in it
+                topics=('clicks', 'users'),
+                task_name='source1',
             ) as stream:
                 async for value in stream:
                     print(f"Subscriber received {value}")