Fix incorrect argument description in on_trace_end docstring (openai#958)

The docstring for the `on_trace_end` method in the `TracingProcessor`
class incorrectly describes the argument `trace`. The current
description states that the argument is the trace that started, but it
should refer to the trace that has finished. This commit updates the
docstring to clarify that the argument represents the trace that has
completed, ensuring accuracy and clarity.

Changes:
- Updated the argument description for `trace` in `on_trace_end` to
clarify it refers to the trace that finished.

Author: shirazkk

src/agents/tracing/processor_interface.py

@@ -23,7 +23,7 @@ def on_trace_end(self, trace: "Trace") -> None:
         """Called when a trace is finished.
 
         Args:
-            trace: The trace that started.
+            trace: The trace that finished.
         """
         pass