Change definition for Session.GradingPeriod

Summary of outcomes for Data Standard 2.1

• Problem Addressed: Current annotation for Session.GradingPeriod is misleading.
• High-level Reasoning: The current annotation for Session.GradingPeriod reads "Grading periods associated with the calendar." However, a Calendar entity was introduced for version 2.1, which may cause a misconception that the annotation seem as though it is referring to the Calendar domain entity, which it is not.
• Changes Made: Update annotation for Session.GradingPeriod to read "Grading periods associated with the session."

Original ticket description follows:

Current definition states: "Grading periods associated with the calendar."

For clarity (since there is now a Calendar entity), the definition should be changed to

"Grading periods associated with the session."