Improve fastapi instrumentation example (open-telemetry#3372)

Author: andremm

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

@@ -64,6 +64,10 @@ async def foobar():
 
 .. code-block:: python
 
+    from opentelemetry.instrumentation.fastapi import FastAPIInstrumentor
+    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")
@@ -76,7 +80,7 @@ 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")
 
-   FastAPIInstrumentor().instrument(server_request_hook=server_request_hook, client_request_hook=client_request_hook, client_response_hook=client_response_hook)
+    FastAPIInstrumentor().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
 *****************************************