Improve tornado instrumentation example (open-telemetry#3410)

Co-authored-by: Riccardo Magliocchetti <[email protected]>

Author: andremm

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

@@ -72,6 +72,8 @@ def get(self):
 
 .. code-block:: python
 
+    from opentelemetry.instrumentation.tornado import TornadoInstrumentor
+
     # will be called for each incoming request to Tornado
     # web server. `handler` is an instance of
     # `tornado.web.RequestHandler`.
@@ -94,7 +96,7 @@ def client_response_hook(span, future):
     TornadoInstrumentor().instrument(
         server_request_hook=server_request_hook,
         client_request_hook=client_request_hook,
-        client_response_hook=client_response_hook
+        client_response_hook=client_response_hook,
     )
 
 Capture HTTP request and response headers