Adjust function name in docs for Message.__len__ function

[hcmturner]

The docs currently describe a Message.len() method. This method does not exist, the method implemented is the normal Message.len method, which then provides len(Message). Change is to rename the function in the docs to len so that it is clear that the built in len method is the one being created.

Caught me out recently, and has previously caught someone out: https://github.com/confluentinc/confluent-kafka-python/issues/1142

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