mutt: Omit seconds in attribution time

Also rewrite RFC comment a bit to reflect better that the date/time is
not compliant (it hasn't been before either, but now even more).

.config/neomutt/muttrc

@@ -4,8 +4,8 @@ source ~/.config/neomutt/accounts.rc
 
 # Shorter date-time format without seconds or timezone
 set date_format='%Y-%m-%d %R'
-# Full RFC 3339 like date in replies
-set attribution='On %[%F] at %[%T %z], %f wrote:'
+# RFC 3339 / ISO 8601 -like date in replies
+set attribution='On %[%F] at %[%R %z], %f wrote:'
 
 # Sort in threads with the most recent message at the top
 set sort=reverse-threads