Use datetime example for the `Field.initial` attribute

Just like regular Django fields, the example adopted uses a `datetiume.date.today` to
illustrate the functionality.

docs/api-guide/fields.md

@@ -84,10 +84,10 @@ A text string that may be used as a description of the field in HTML form fields
 A value that should be used for pre-populating the value of HTML form fields. You may pass a callable to it, just as
 you may do with any regular Django `Field`:
 
-    def user_default_color():
-        return 'blue'
-
-    color = serializers.CharField(initial=user_default_color)
+    import datetime
+    from rest_framework import serializers
+    class ExampleSerializer(serializers.Serializer):
+        day = serializers.DateField(initial=datetime.date.today)
 
 ### `style`