Update IssueEvent.Event assigned/unassigned doc. (#615)
Update IssueEvent.Event assigned/unassigned doc. (#615)

The current phrase reads as if the "actor" field contains the
information about the user who changed the assignment, but this
information is actually in the "assigner" field. The "actor" field has
the same value as the "assignee" field, which is the user that has
been assigned to or unassigned from the issue.

Fix the description of assigned, unassigned events to reflect that.
google/go-github