change DATE_FORMAT to be compatible with Nikola 8

conf.py

@@ -112,7 +112,7 @@ TIMEZONE = "UTC"
 
 # Date format used to display post dates.
 # (str used by datetime.datetime.strftime)
-DATE_FORMAT = '%Y-%m-%d'
+DATE_FORMAT = 'y-M-d'
 
 # Date format used to display post dates, if local dates are used.
 # (str used by moment.js)