Improve starlette instrumentation example (open-telemetry#3409)

Author: andremm

instrumentation/opentelemetry-instrumentation-starlette/src/opentelemetry/instrumentation/starlette/__init__.py

@@ -62,6 +62,10 @@ def home(request):
 
 .. code-block:: python
 
+    from opentelemetry.instrumentation.starlette import StarletteInstrumentor
+    from opentelemetry.trace import Span
+    from typing import Any
+
     def server_request_hook(span: Span, scope: dict[str, Any]):
         if span and span.is_recording():
             span.set_attribute("custom_user_attribute_from_request_hook", "some-value")
@@ -74,7 +78,7 @@ def client_response_hook(span: Span, scope: dict[str, Any], message: dict[str, A
         if span and span.is_recording():
             span.set_attribute("custom_user_attribute_from_response_hook", "some-value")
 
-   StarletteInstrumentor().instrument(server_request_hook=server_request_hook, client_request_hook=client_request_hook, client_response_hook=client_response_hook)
+    StarletteInstrumentor().instrument(server_request_hook=server_request_hook, client_request_hook=client_request_hook, client_response_hook=client_response_hook)
 
 Capture HTTP request and response headers
 *****************************************