fix(docs): use correct image for custom course banner (openedx#796)

Co-authored-by: Adolfo R. Brandes <adolfo@axim.org>

Author: brian-smith-tcril

src/slots/CourseBannerSlot/README.md

@@ -17,7 +17,7 @@ The default CourseBanner looks like this when audit access has expired for the c
 
 The following configuration will render a custom implementation of a CourseBanner under every `CourseCard`.
 
-![Screenshot of custom banner added under CourseCard](./images/course_banner_slot_default.png)
+![Screenshot of custom banner added under CourseCard](./images/custom_course_banner.png)
 
 ```js
 import { WidgetOperationTypes } from '@openedx/frontend-base';