Change text in the LMS Dashboard Roster popup info

**The problem**
The frequency of our roster updates are discussed in our KB, but are not always apparent to users. There's the possibility, especially during the add/drop period schools often have in the first 2 weeks of an academic term, where a student could join a class where we've already roster synced, complete a Hypothesis assignment, and will not have been added to the roster by the time an instructor checks the dashboard.

While other technical solutions may help with this, for now additional explanation text related to the roster sync information can help users understand why some students may not be listed on the dashboard yet.

**Proposed solution**
Add the following text to the pop up (possibly on a new line): "Hypothesis re-syncs with LMS rosters every three days."

No implementation file or test is identified in this product-backlog issue. Start by locating the LMS Dashboard roster popup in the product repository, then verify that the proposed sentence appears in the popup and explains the three-day LMS roster resync interval.

Written by the indexing model from the issue text.