Improve wsgi instrumenation example (#3577)

Author: andremm

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

@@ -97,6 +97,7 @@ def GET(self):
 
 .. code-block:: python
 
+    from opentelemetry.trace import Span
     from wsgiref.types import WSGIEnvironment, StartResponse
     from opentelemetry.instrumentation.wsgi import OpenTelemetryMiddleware