docs: correct mistake in imaplib example

This is a correction to 8077f2e, 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.

Author: foresto

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')