[docs] [issue16071] fix link to email.message.Message in mailbox docs

[Kushal Das]

Kushal Das added the comment:

But the whole 18.4.2 is mount mailbox.Message so, it seems to be correct documentation. 
email.Message.Message is properly mentioned in the 4th line properly.
I don't think it is bug.

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nosy: +kushaldas

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<http://bugs.python.org/issue16071>
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