fix typo

Doc/library/ftplib.rst

@@ -315,7 +315,7 @@ followed by ``lines`` for the text version or ``binary`` for the binary version.
 .. method:: FTP.quit()
 
    Send a ``QUIT`` command to the server and close the connection. This is the
-   "polite" way to close a connection, but it may raise an exception of the server
+   "polite" way to close a connection, but it may raise an exception if the server
    responds with an error to the ``QUIT`` command.  This implies a call to the
    :meth:`close` method which renders the :class:`FTP` instance useless for
    subsequent calls (see below).