Fix an interval of a due date

Author: junichi11

src/main/java/com/junichi11/netbeans/modules/github/issues/issue/GitHubIssue.java

@@ -316,7 +316,7 @@ public Date getDueDate() {
             return null;
         }
         calendar.setTime(date);
-        calendar.add(Calendar.DATE, interval - 1);
+        calendar.add(Calendar.DATE, interval);
         return calendar.getTime();
     }