Updated timezone docs to use timezone.now()

Thanks James Cleveland for the report and Aymeric for suggesting
the solution.

docs/topics/i18n/timezones.txt

@@ -78,20 +78,13 @@ the current time, you would write::
 
     now = datetime.datetime.now()
 
-When time zone support is enabled, Django uses time-zone-aware datetime
-objects. If your code creates datetime objects, they should be aware too. In
-this mode, the example above becomes::
+When time zone support is enabled (:setting:`USE_TZ=True <USE_TZ>`), Django uses
+time-zone-aware datetime objects. If your code creates datetime objects, they
+should be aware too. In this mode, the example above becomes::
 
-    import datetime
-    from django.utils.timezone import utc
-
-    now = datetime.datetime.utcnow().replace(tzinfo=utc)
-
-.. note::
+    from django.utils import timezone
 
-    :mod:`django.utils.timezone` provides a
-    :func:`~django.utils.timezone.now()` function that returns a naive or
-    aware datetime object according to the value of :setting:`USE_TZ`.
+    now = timezone.now()
 
 .. warning::
 
@@ -554,7 +547,7 @@ Troubleshooting
        >>> import datetime
        >>> from django.utils import timezone
        >>> naive = datetime.datetime.utcnow()
-       >>> aware = naive.replace(tzinfo=timezone.utc)
+       >>> aware = timezone.now()
        >>> naive == aware
        Traceback (most recent call last):
        ...