Fix missing step in docs (open-telemetry#1113)

As far as I can tell, the tutorial in the docs doesn't actually work
without this line.

Author: alertedsnake

Diff for: instrumentation/opentelemetry-instrumentation-grpc/src/opentelemetry/instrumentation/grpc/__init__.py

@@ -106,6 +106,7 @@ def SayHello(self, request, context):
     def serve():
 
         server = grpc.server(futures.ThreadPoolExecutor())
+        server = intercept_server(server, server_interceptor())
 
         helloworld_pb2_grpc.add_GreeterServicer_to_server(Greeter(), server)
         server.add_insecure_port("[::]:50051")