Doc: Use the `with` statement in the first example of the ftplib doc. (GH-16271)

Doc/library/ftplib.rst

@@ -33,7 +33,8 @@ Here's a sample session using the :mod:`ftplib` module::
    drwxr-sr-x    4 1176     1176         4096 Nov 17  2008 project
    drwxr-xr-x    3 1176     1176         4096 Oct 10  2012 tools
    '226 Directory send OK.'
-   >>> ftp.retrbinary('RETR README', open('README', 'wb').write)
+   >>> with open('README', 'wb') as fp:
+   >>>     ftp.retrbinary('RETR README', fp.write)
    '226 Transfer complete.'
    >>> ftp.quit()