Fix +calendar/open-calendar

It was using a doom-themes function, which isn't guaranteed to exist,
e.g. if the user isn't using a doom-themes theme.

modules/app/calendar/autoload.el

@@ -42,7 +42,7 @@
    ;; :custom-map cfw:my-cal-map
    :contents-sources
    (list
-    (cfw:org-create-source (doom-color 'fg))  ; orgmode source
+    (cfw:org-create-source (face-foreground 'default))  ; orgmode source
     )))
 
 ;;;###autoload