Adjust function name in docs for Message.__len__ function

[hcmturner]

Adjust the docstring so it is clear the function is the built in __len__ function and not some newly defined len function on this object.

[cla-assistant]

All committers have signed the CLA.

[hcmturner]

How do I go about getting code owner review here?

[confluent-cla-assistant]

Please sign the Contributor License Agreement here before this PR can be approved.
❌ hcmturner
_{Please push an empty commit if you would like to re-run the checks to verify CLA status for all contributors.}

[joaoe]

How do I go about getting code owner review here?

You don't. They haven't been accepting PRs for a couple years. My PRs are stuck.