docs: update course_unit_sidebar slot ID to v2 in plugin-slots README (#2905)

Co-authored-by: Claude Sonnet 4.6 <[email protected]>

Author: brian-smith-tcril

src/plugin-slots/README.md

@@ -8,7 +8,7 @@
 
 ## Course Unit page
 * [`org.openedx.frontend.authoring.course_unit_header_actions.v1`](./CourseUnitHeaderActionsSlot/)
-* [`org.openedx.frontend.authoring.course_unit_sidebar.v1`](./CourseAuthoringUnitSidebarSlot/)
+* [`org.openedx.frontend.authoring.course_unit_sidebar.v2`](./CourseAuthoringUnitSidebarSlot/)
 
 ## Other Slots
 * [`org.openedx.frontend.authoring.additional_course_content_plugin.v1`](./AdditionalCourseContentPluginSlot/)