Fix spelling: an handler -> a handler

Thanks @rastamanjohn for the hint

docs/source/api/handlers.rst

@@ -4,7 +4,7 @@ Update Handlers
 Handlers are used to instruct Pyrogram about which kind of updates you'd like to handle with your callback functions.
 
 For a much more convenient way of registering callback functions have a look at `Decorators <Decorators.html>`_ instead.
-In case you decided to manually create an handler, use :meth:`add_handler() <pyrogram.Client.add_handler>` to register
+In case you decided to manually create a handler, use :meth:`add_handler() <pyrogram.Client.add_handler>` to register
 it.
 
 .. code-block:: python

docs/source/start/updates.rst

@@ -15,8 +15,8 @@ using `Handlers <../api/handlers>`_.
 Each handler deals with a specific event and once a matching update arrives from Telegram, your registered callback
 function will be called back by the framework and its body executed.
 
-Registering an Handler
-----------------------
+Registering a Handler
+---------------------
 
 To explain how handlers work let's have a look at the most used one, the
 :obj:`MessageHandler <pyrogram.MessageHandler>`, which will be in charge for handling :obj:`Message <pyrogram.Message>`