Fix @stream typo in usage-rules liveview streams example (#6601)

The empty-state example used `@stream.tasks` (singular) but it should
be `@streams.tasks` (plural) to match the streams API — consistent
with every other example in the file.

Author: tommeier

usage-rules/liveview.md

@@ -37,7 +37,7 @@
 
       <div id="tasks" phx-update="stream">
         <div class="hidden only:block">No tasks yet</div>
-        <div :for={{id, task} <- @stream.tasks} id={id}>
+        <div :for={{id, task} <- @streams.tasks} id={id}>
           {task.name}
         </div>
       </div>