Improve django instrumentation example (#3313)

Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>
Co-authored-by: Leighton Chen <lechen@microsoft.com>

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

@@ -129,6 +129,8 @@ The hooks can be configured as follows:
 
 .. code:: python
 
+    from opentelemetry.instrumentation.django import DjangoInstrumentor
+
     def request_hook(span, request):
         pass