Added import in docs/topics/email.txt example.

docs/topics/email.txt

@@ -279,6 +279,8 @@ All parameters are optional and can be set at any time prior to calling the
 
 For example::
 
+    from django.core.mail import EmailMessage
+
     email = EmailMessage('Hello', 'Body goes here', 'from@example.com',
                 ['to1@example.com', 'to2@example.com'], ['bcc@example.com'],
                 reply_to=['another@example.com'], headers={'Message-ID': 'foo'})