Update labeling on video views per time segment graph on all three course-level dashboards

[crathbun428]

User Story
As a course delivery team member, I want to add helper text and update the name of the Watched Video Segments graph, so that the contents of the graph are clear to users and they know how to filter the graph to show watched video segments for one video.

Acceptance Criteria
Given I have access to the Course, Individual and At-Risk Learner dashboards
When I view the Watched Video Segments Graph on the three course-level dashboards dashboard
Then I see that the name of the graph is updated to Number of Views across Video Duration
And I see helper text added to the graph that reads: To view the number of views across the duration of a single video, click the name of the video in the Video Name and Location column of the Video Engagement table above.

UX/UI Direction
See screenshot below (please note: the graph label capitalization should look like this on the chart: Number of Views across Video Duration

[crathbun428]

This issue is complete.