[daggy-u] include SensorEvaluationContext import in full code example (…

…#23029)

## Summary & Motivation

- Includes SensorEvaluationContext import in full example at end of
"Lesson 9: Building the sensor"

Comment from community member, Joseph:
> Hi, I'm going through the Dagster Essentials course and noticed the
SensorEvaluationContext import is missing from the last code block of
[Building the
sensor](https://courses.dagster.io/courses/take/dagster-essentials/multimedia/48127179-building-the-sensor).
Just thought y'all should know

docs/dagster-university/pages/dagster-essentials/lesson-9/building-the-sensor.md

@@ -157,6 +157,7 @@ Putting everything together, you should have the following code in `sensors/__in
 ```python
 from dagster import (
     RunRequest,
+    SensorEvaluationContext,
     SensorResult,
     sensor,
 )