docs: changed user_name to username in docstrings

redmail/email/__init__.py

@@ -23,7 +23,7 @@ def send_email(*args, host:str, port:int, username:str=None, password:str=None,
         User to send the email with
     password : str
         Password of the user to send the email with
-    user_name : str
+    username : str
         Deprecated alias for username. Please use username instead.
     **kwargs : dict
         See redmail.EmailSender.send

redmail/email/sender.py

@@ -241,7 +241,7 @@ class EmailSender:
                 email = EmailSender(
                     host='localhost', 
                     port=0, 
-                    user_name='me@example.com', 
+                    username='me@example.com', 
                     password='<PASSWORD>'
                 )
                 email.send(