docs: correct mistake in imaplib example

This is a correction to 8077f2eab2, which
changed a variable name in only one place and broke the subsequent
reference to it, departed from the naming convention used in the rest of
the module, and shadowed the type() builtin along the way.

Doc/library/imaplib.rst

@@ -334,7 +334,7 @@ An :class:`IMAP4` instance has the following methods:
    Example::
 
       with M.idle(dur=29 * 60) as idler:
-          for type, datum in idler:
+          for typ, datum in idler:
               print(typ, datum)
 
       ('EXISTS', b'1')