clean up old changelog and docstring

Author: keith-decker

instrumentation/opentelemetry-instrumentation-langchain/.changelog/36.added

@@ -26,7 +26,6 @@ class RetrievalInvocation(GenAIInvocation):
     """Represents a single retrieval invocation (retrieval span).
 
     Use handler.retrieval() rather than constructing this directly.
-    rather than constructing this directly.
 
     Reference: https://github.com/open-telemetry/semantic-conventions/blob/main/docs/gen-ai/gen-ai-spans.md#retrievals