Fix the board list label field

It needs to be a single label, not a slice of labels.

boards.go

@@ -49,7 +49,7 @@ func (b IssueBoard) String() string {
 // GitLab API docs: https://docs.gitlab.com/ce/api/boards.html
 type BoardList struct {
 	ID       int    `json:"id"`
-	Labels   []*Label `json:"labels"`
+	Label    *Label `json:"label"`
 	Position int    `json:"position"`
 }