fix logic error in list-weeks to remove duplicate week

Change-Id: Id3b7c0133b5801670dd9d2b275634cc9701f133e
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tools/list_weeks.py

@@ -42,7 +42,9 @@ while current < next_release_date:
 n_weeks = len(weeks)
 
 # Add the list of Mondays following the release leading up to the
-# summit.
+# summit. Increment current before entering the loop because we've
+# already used that week.
+current += week
 while current <= summit_date:
     weeks.append(current)
     current += week